Organic electroluminescent device

ABSTRACT

An organic electroluminescent device includes at least two light-emissive units provided between a cathode electrode and an anode electrode opposed to the cathode electrode, each of the light-emissive units including at least one light-emissive layer. The light-emissive units are partitioned from each other by at least one charge generation layer, the charge generation layer being an electrically insulating layer having a resistivity of not less than 1.0×10 2  Ωcm.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a Continuation of application Ser. No.14/716,114, filed May 19, 2015, which is a divisional of applicationSer. No. 13/892,868, filed May 13, 2013, which is a continuation ofapplication Ser. No. 13/297,359, filed Jan. 30, 2012 which issued asU.S. Pat. No. 8,482,193 on Jul. 9, 2013, which is a continuation ofapplication Ser. No. 11/732,348, filed Apr. 3, 2007 which issued as U.S.Pat. No. 8,080,934 on Dec. 20, 2011, which is a divisional of abandonedapplication Ser. No. 10/393,952, filed Mar. 21, 2003, all of which claimpriority to Japanese Application No. 200286599 filed Mar. 26, 2002, andJapanese Application No. 200370135 filed Mar. 14, 2003, all of which areincorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to an organic electroluminescent device(hereinafter, abbreviated as an “organic EL device” or “device”) whichcan be used in planar light sources and display devices. Thisapplication is a CON of Ser. No. 14/716,114 May 19, 2015 which is a DIVof Ser. No. 13/892,868 May 13, 2013 U.S. Pat. No. 9,070,892 which is aCON of Ser. No. 13/297,359 Jan. 30, 2012 U.S. Pat. No. 8,482,193 whichis a CON of Ser. No. 11/732,348 Apr. 3, 2007 U.S. Pat. No. 8,080,934which is a DIV of Ser. No. 10/393,952 Mar. 21, 2003 ABN.

2. Description of the Related Art

Recently, attention has been drawn to organic electroluminescent deviceshaving a light-emitting or luminescent layer including an organiccompound between a cathode electrode and an anode electrode opposed tothe cathode electrode as a large area display device operable at a lowdriving voltage. For the purpose of higher efficiency in an EL device,Tang et al., as is disclosed in Appl. Phys. Lett., 51, 913 (1987), havesuccessfully achieved a sufficiently high luminance and high efficiencyfor practice use, i.e., a luminance of 1,000 cd/m² and an externalquantum efficiency of 1% at an applied voltage no more(greater) than 10volts, by adopting a structure in which organic compound layers havingdifferent carrier transporting properties are laminated to therebyintroduce holes and electrons with good balance from an anode and acathode, respectively, and by having the thickness of the organiccompound layer no more(greater) than 2,000 Å.

Furthermore, according to the disclosures of the patents invented byTang et al., (such as Japanese Laid-open Patent Application Nos.59-194393, 63-264692 and 2-15595 and U.S. Pat. Nos. 4,539,507, 4,769,292and 4,885,211) it is stated that if a total layer thickness of theorganic layers sandwiched between an anode and a cathode does not exceedabout 1 μm, an EL device capable of emitting light at a lower level ofthe applied voltage can be provided, and that desirably, if the totallayer thickness is reduced to a range of 1,000 to 5,000 Å, an electricfield (V/cm) useful in obtaining a light emission at an applied voltageno more than about 25 volts can be obtained.

The reason why Tang et al. have directed their attention to a reductionof the layer thickness of the organic layers in attaining a reduction ofthe driving voltage, as described in the above-referenced article,resides in overcoming the problem suggested by Helfrich et al. in the1960s. Namely, Helfrich et al. have observed that an external quantumefficiency of about 5% can be obtained when a sufficient electric fieldelectroluminescence (EL) is applied to an anthracene single crystal;however, according to their method, only a low power conversionefficiency (w/w) could be obtained, since the voltage required to drivesuch devices is quite high (greater than 100V).

Referring to the above-reference Tang et al. patents, the organic ELdevices suggested therein have a multilayered structure in which ananode, a hole injection (transporting) layer, a light-emitting layer(having an electron transporting property) and a cathode are laminatedin that order, and the devices can provide a quantum efficiency of atleast about 5×10⁻⁴ (0.05%). Furthermore, the quantum efficiency isdefined in Japanese Laid-open Patent Application No. 59-194393 as the ELquantum efficiency simply equaling the ratio of photons per secondemitted from the cell, to the electrons per second measured in theexternal circuit.

Presently, as has been already disclosed, when a fluorescent material(utilizing emission from a singlet excitation state) is used in the thinlayer EL devices suggested by Tang et al., a quantum efficiency above 5%can be obtained. Furthermore, when a phosphorescent material (utilizingemission from a triplet excitation state) is used in the EL devices, aquantum efficiency approaching to 20% can be obtained.

As can be appreciated from the above description, the quantum efficiencyis calculated from the number of the photons actually emitted from(outside of) the device, and thus the quantum efficiency is calledexternal quantum efficiency. On the other hand, the number of photonsgenerated internally in the device might be quite large when comparedwith the value observed externally, and it is predicted that suchefficiency, called internal quantum efficiency might reach about 5 timesof the external quantum efficiency. Accordingly, even presently, whenusing a phosphorescent material, an internal quantum efficiency can beexhibited at 100%, and thus it seems that the remaining problem in theorganic EL devices resides only in an increase of the reliabilityconcerning the operational life-time of the devices.

As described above, the suggestions by Tang et al. in their patents andarticles have accelerated a worldwide research and development in thefield of organic EL devices, and thus a great number of improved ELdevices have been developed based on the basic device structuresuggested by Tang et al. Presently, commercialization of the EL deviceshas already started in regard to their use as a display device on adashboard or in a cellular phone.

However, from a viewpoint of durability of the device, theabove-described conventional organic EL devices can barely attain ahalf-decay life time exceeding 10,000 hours with a luminance of only theorder of 100 cd/m², which is required in display use. Presently, it isstill difficult to attain a required practical operational life-time(10,000 hours or more) with a luminance of about 1,000 to 10,000 cd/m²,which is required in illumination use, etc. In fact, an organic ELdevice having a high luminance and long operational life-time is stillnot realized and not commercially available.

As described above, attention that has recently been drawn to organic ELdevices has been based on the discovery of a thin film-forming materialwhich drives the resulting device at a low voltage of not more than 10volts. However, the resulting device still suffers from the disadvantagethat if the device is intended to obtain a high luminance emissionnecessary for illumination purposes, a higher current densityapproaching tens of mA/cm² to hundreds of mA/cm² is necessary. Note thatin the best green light-emitting devices currently available, aluminance of about thousands to tens of thousands of cd/m² still needsthe above-mentioned current density of about 10 to 100 mA/cm². It can beconsidered that this property is characteristic of charge injection typedevices (like this organic EL device), and such characteristics cancause a relatively large problem with the operational life-time oforganic EL devices in comparison with an inorganic LED (light-emittingdiode) which is also a charge injection device and uses an inorganiccompound semiconductor which can be more robust than organic compounds.

In an organic layer formed from a low molecular organic material via avacuum vapor deposition method, the nature of the electric currentpassing through the organic layer is defined as a hopping conduction ofelectrons and holes between the molecules of the material. Furthermore,when observing the molecules from the chemical aspect, it can bedescribed like this; the electron transporting molecules and the holetransporting molecules which are generally being as electrically neutralmolecules are repeatedly subjected to a process in which the electrontransporting and hole transporting molecules are shifted to a radicalanion state or a radical cation state, i.e., the oxidation-reductionreaction in terms of Lewis' chemistry is being repeated between thesemolecules. Referring to the above-described property in the organic ELdevices, i.e., that a higher current density is required to attainhigher luminance, this property means that the oxidation-reductionreactions are repeated at a higher frequency. Obviously, thedeterioration speed of the organic molecules is proportional to afrequency of the oxidation-reduction reactions, namely, the currentdensity.

To solve the above problem, Japanese Laid-open Patent Application No.11-329748 (corresponding U.S. Pat. No. 6,107,734) suggests an organic ELdevice in which a plurality of organic light-emitting layers areelectrically connected in series through an intermediate conductivelayer, and with regard to the intermediate conductive layer, describesthat many types of materials may be used in the formation of theintermediate conductive layer, as long as they (the intermediateconductive layer) are capable of injecting holes and electrons to one orthe other primary surface side, and capable of keeping an approximateequipotential in the layer.

This EL device, however, suffers from the following problem. Forinstance, in the display device having a simple matrix structure, thelight emission area upon voltage application should be defined only tothe pixel, i.e., the intersection area, sandwiched by cathode and anodeline, thereby enabling to display a motion picture. However, in theabove-described case in which the intermediate conductive layer having asubstantially equipotential surface is formed in a substantially overallsurface in an area which is equal to the area of the organiclight-emitting layers, i.e., when the intermediate conductive layer isalso formed in areas other than the intersection area sandwiched bycathode and anode line, light emission can be generated in areas otherthan the intersection areas in which the light emission is desired to begenerated. Specifically, there is a possibility of generating lightemission in all of the crossed area of the cathode with the intermediateconductive layer, the crossed area of the anode and with theintermediate conductive layer, and if two or more intermediateconductive layers are contained, the crossed area between oneintermediate conductive layer and another intermediate conductive layer.

Accordingly, it is described in Japanese Laid-open Patent ApplicationNo. 11-329748 that the intermediate conductive layers of each pixel areseparated not only from the intermediate conductive layer of theadjacent pixels, but also from a power source. Furthermore, one idea toseparate the intermediate conductive layers from each other in thepixels in the EL device having a simple matrix structure is alsodescribed in this publication. If an interlayer insulation film ispreviously formed and disposed at a layer thickness above 1 μm and inthe form of a sharp step pattern, the conductive layer can beautomatically separated in the presence of a suddenly-changed shape ofthe interlayer insulation film, even if the conductive layer is formedusing the shadow mask identical to the one for an organic materialdeposition.

However, in this case, although the cathode should not be separated, thecathode can be separated by the interlayer insulation film if thecathode has only a thickness of about 0.1 μm (100 nm) as in theconventional organic EL devices. To avoid this problem, JapaneseLaid-open Patent Application No. 11-329748 teaches use of In (indium) asthe cathode material at large thickness, thereby preventing electricalseparation of the cathode line, because indium cannot easily causeproblems due to crystallization (this problem is generally referred toas “hillock”), even if the cathode is formed at a thickness of 1 μm ormore.

In this alternative case, however, a problem of the throughput reductioncannot be also avoided, because a metal such as Al (aluminum), which isa conventional and low-cost wiring material, cannot be used as a cathodematerial and also it is necessary to stably form “an interlayerinsulation film having a layer thickness above 1 μm and asuddenly-changed shape of the interlayer”.

Furthermore, the inventors of the present invention have also proposedanother organic EL device in Japanese Patent Application No.2001-225847, and has at least two light-emitting units constituting theconventional organic EL device (the components in all the elementsconstituting the conventional organic EL device except for a cathode andan anode), and the contained light-emitting units are separated fromeach other with a transparent layer acting as an equipotential surface.

The “equipotential surface” used herein means that when a voltage isapplied, the transparent layer cannot exhibit a substantial potentialdifference in both a thickness direction and a planar (lateral)direction in the layer. In other words, although the inventors have notspecifically disclosed, they have implied the necessity to construct theequipotential surface from an electrically conductive material, i.e.,any material having a resistivity less than 1.0×10² Ωcm.

However, as in the above-discussed Japanese Laid-open Patent ApplicationNo. 11-329748, if the two or more light-emitting units are separatedusing a material having a high electrical conductivity (low resistivity)described above, there may be difficulties in defining light emissionareas as required, due to the conductivity in a planar (lateral)direction (direction parallel to a substrate).

In practice, as shown in FIG. 38B, even if the production of the ELdevice is carried out in accordance with the method of JapaneseLaid-open Patent Application No. 11-329748 by producing a cathode 55 andan anode 52, both in the form of a strip having a width of 2 mm, andarranging the cathode 55 and the anode 52 so that they are crossed atright angles, thereby producing a light emission area corresponding tothe crossed (intersection) area, i.e., 2 mm square (□), unexpected lightemission may be caused in other areas when there is an area having anequipotential surface 54 is extended to another area. The undesirableemission in the EL device is shown in the photograph of FIG. 38A.

To avoid the above problem, as disclosed in the examples of JapanesePatent Application No. 2001-225847, the inventors had to form anequipotential surface using a shadow mask (2 mm square pattern; □)having a patterned opening which corresponds to the desired lightemission area, thereby selectively forming the equipotential surfaceonly in the desired emission layer. However, in this method, it isdifficult to attain selective emission only in the desired pixels in thedisplay device, because the display device has to be produced at a pixellength and pitch (between each pixel) of about 0.1 mm or less.

In regard to improving productivity in mass-production of the ELdevices, frequent changing and precise positioning operations of theshadow mask is not desirable, because it causes tremendous reduction ofthroughput.

SUMMARY OF THE INVENTION

In view of the above problems in the conventional organicelectroluminescent (EL) devices, the present invention provides anorganic EL device which can effectively and stably provide a devicestructure capable of achieving a long operational life time with alight-emission at a higher luminance, which cannot be easily attained inconventional EL devices. In the production of such organicelectroluminescent (EL) devices, the formation of two or morelight-emissive units (mainly formed from an organic material),sandwiched between a cathode and an anode, frequent change and precisepositioning of shadow masks for defining a deposition area is notrequired during the formation of a charge generation layer, which isnewly introduced in the present invention. A formation of the interlayerinsulation film in a sudden-changed shape which has a risk of causingdisconnection of a cathode line is also not required, thus enabling toenhance productivity and to simplify the process of manufacturing simplematrix-type display devices, etc. According to an aspect of the presentinvention, an organic electroluminescent device is provided, includingat least two light-emissive units provided between a cathode electrode,and an anode electrode opposed to the cathode electrode, each of thelight-emissive units including at least one light-emissive layer. Thelight-emissive units are partitioned from each other by at least onecharge generation layer, the charge generation layer constituting anelectrically insulating layer having a resistivity of not less than1.0×10²Ω cm.

It is desirable for the charge generation layer to constitute anelectrically insulating layer having a resistivity of not less than1.0×10 Ωcm.

It is desirable for the charge generation layer to include a laminatedand/or mixed layer formed from two different materials. A chargetransfer complex including a radical cation and a radical anion isformed upon an oxidation-reduction reaction between the two materials,and a radical cation state and a radical anion state in the chargetransfer complex is transferred to a direction of the cathode and adirection of the anode, respectively, when a voltage is applied to thedevice, so that a hole is injected into the light-emissive unit which islocated on a cathode side of the charge generation layer and is adjacentthereto, and an electron is injected into the light-emissive unit whichis located on an anode side of the charge generation layer and isadjacent thereto.

It is desirable for the charge generation layer to include a laminatedand/or a mixed layer including an organic compound having an ionizationpotential of less than 5.7 eV and a hole transporting property orelectron donating property; and an inorganic and/or organic materialcapable of forming a charge transfer complex through theoxidation-reduction reaction thereof with the organic compound. Thecharge generation layer contains a charge transfer complex formed uponthe oxidation-reduction reaction between the organic compound and one ofan inorganic and organic material.

The organic compound can include an arylamine compound, wherein thearylamine compound is represented by the following formula (I):

wherein Ar1, Ar2 and Ar3 each independently represents an aromatichydrocarbon group which may have substituents.

It is desirable for the organic compound to include an arylaminecompound having a glass transition temperature of not lower than 90° C.

The arylamine can include one of α-NPD, 2-TNATA, spiro-TAD, andspiro-NPB.

The inorganic material can be a metal oxide.

The inorganic material can be a metal halide.

The metal oxide can be vanadium pentaoxide or rhenium heptaoxide.

The inorganic material can be deposited by one of a resistive heatingvapor deposition method, an electron beam vapor deposition method and alaser beam vapor deposition method.

The inorganic material can be deposited by a sputtering method. Asputtering apparatus used in the sputtering method is a facing targetsputtering system which includes a pair of opposed targets provided at acertain distance, a reflection electrode capable of reflecting electronstowards a front peripheral area of each target, and a magnetic fieldgeneration device capable of forming a parallel magnetic field in thevicinity of the peripheral portion of each target, the magnetic fieldhaving a portion parallel to the peripheral portion of the target.

The organic material can include at least one fluorine as a substituentgroup, and possess at least one of an electron injection property and anelectron accepting property.

The organic material can include at least one cyano group as asubstituent group, and possess at least one of an electron injectionproperty and an electron accepting property.

The organic material can be tetrafluoro-tetracyanoquinodimethane(4F-TCNQ).

The light-emissive unit can include, as a layer located on an anode sideof the charge generation layer and being adjacent thereto, an electroninjection layer having a mixture including an organic compound and ametal functioning as an electron donating dopant.

The electron donating dopant can include at least one metal selectedfrom a group including an alkaline metal, an alkaline earth metal and arare earth metal.

The metal of the electron donating dopant can be provided in a molarratio of 0.1 to 10 with respect to the organic compound in the electroninjection layer.

The light-emissive unit can include, as a layer located on an anode sideof the charge generation layer and being adjacent thereto, a metal layerhaving a thickness of not more than 5 nm formed from a metal selectedfrom an alkaline metal, an alkaline earth metal and a rare earth metal.The metal constituting the layer diffuses in the adjacent electrontransporting layer to react with electron transporting organic material.As a result of the diffusion, an electron injection layer being composedof a mixture including the electron transporting organic material and ametal functioning as an electron donating dopant is formed.

The light-emissive unit can include, as a layer located on an anode sideof the charge generation layer and being adjacent thereto, a layerincluding an organic metal complex compound including at least one metalion selected from an alkaline metal ion, an alkaline earth metal ion anda rare earth metal ion, and a reaction generating layer which is formedby an in-situ reduction reaction when a thermally reducible metal, whichcan reduce a metal ion in the organic metal complex to a metal in avacuum is deposited on the organic metal complex constituting the layer.

The light-emissive unit can include, as a layer located on an anode sideof the charge generation layer and being adjacent thereto, a layerincluding an inorganic compound including at least one metal ionselected from an alkaline metal ion, an alkaline earth metal ion and arare earth metal ion, and a reaction generating layer which is formed byan in-situ reduction reaction when a thermally reducible metal, whichcan reduce a metal ion in the inorganic compound to a metal in a vacuumis deposited on the inorganic compound constituting the layer.

It is desirable for the thermally reducible metal to include at leastone selected from Aluminum, Zirconium, Silicon, Titanium and Tungsten.

The light-emissive unit can include a structure, as a layer located onan anode side of the charge generation layer and being adjacent thereto,in which a layer of a mixture including an organic compound and anelectron donating dopant is formed, thereafter, a reaction generatinglayer is generated by an in-situ reduction reaction when a thermallyreducible metal, which can reduce an alkaline metal ion, an alkalineearth metal ion or a rare earth metal ion to a metal in a vacuum, isdeposited on an organic metal complex compound containing at least onemetal ion selected from an alkaline metal ion, an alkaline earth metalion and a rare earth metal ion.

The light-emissive unit can include a structure, as a layer located onan anode side of the charge generation layer and being adjacent thereto,in which a layer of a mixture including an organic compound and anelectron donating dopant is formed, a reaction generating layer isgenerated by an in-situ reduction reaction when a thermally reduciblemetal, which can reduce an alkaline metal ion, an alkaline earth metalion or a rare earth metal ion to a metal in a vacuum, is deposited on aninorganic compound containing at least one metal ion selected from analkaline metal ion, an alkaline earth metal ion and a rare earth metalion.

The light-emissive unit can include, as a layer located on a cathodeside of the charge generation layer and being adjacent thereto, a holeinjection layer including a mixture of an organic compound and anelectron accepting compound having a property capable of oxidizing theorganic compound in terms of Lewis acid chemistry.

The electron accepting compound having a property capable of oxidizingthe organic compound in the hole injection layer in terms of Lewis acidchemistry can be provided in a molar ratio of 0.01 to 10 with respect tothe organic compound.

The light-emissive unit can include, as a layer located on a cathodeside of the charge generation layer and being adjacent thereto, a holeinjection layer including an electron accepting compound and having athickness of not more than 30 nm.

The light-emissive units can each have different emission spectrums.

The organic electroluminescent device can emit white light due tosuperimposing of different lights from each light-emissive unit.

At least one of the light-emissive units can include a light-emissivelayer containing a phosphorescent material.

In each of the light-emissive units, it is desirable for an optical pathlength from a light-emissive site to a light-reflective metal electrodeto be an odd-numbered times a quarter wavelength of light.

All the layers including the light-emissive units, the charge generationlayer and the electrode layer can be formed on a substrate by heating avaporizable material in a vacuum to deposit one of a vaporized andsublimed material on the substrate. Upon depositing the vaporized orsublimed material on the substrate, a substrate is transported in adirection of a planar surface thereof, a deposition area being open in alower surface of the substrate; a container is provided, in a lowerposition of the transporting substrate, including a vaporizable materialhaving a deposition width which can cover the deposition area extendingin a direction perpendicular to the transportation direction of thesubstrate; and the container is heated to thereby one of vaporize andsublime so as to deposit the vaporizable material provided in thecontainer.

It is desirable for a combined thickness of the light-emissive units andthe charge generation layers, sandwiched between the cathode and theanode, to be greater than 1,000 nm (1 μm).

It is desirable for the organic electroluminescent device to be operatedat a driving voltage of more than 25 volts.

It is desirable for light to be able to be passed in only one directionwhich is one of an anode electrode direction and a cathode electrodedirection, from a light generation site in the organicelectroluminescent device, wherein light advancing in a directionopposite to the only one direction is absorbed by a light-absorbingmedium, and wherein, in each of the light-emissive units, a lightinterference effect is removed so that an adjustment of an optical pathlength from a light-emissive site of the light-emissive layers to alight-reflective metal electrode is substantially not necessary.

It is desirable for light advancing in one direction which is one of ananode electrode direction and a cathode electrode direction, from alight generation site in the organic electroluminescent device to bereflected diffusely by a diffused reflection medium, and in each of thelight-emissive units, a light interference effect to be removed, so thatan adjustment of an optical path length from a light-emissive site ofthe light-emissive layers to a light-reflective metal electrode issubstantially not necessary.

The present disclosure relates to subject matter contained in JapanesePatent Application Nos. 2002-86599 (filed on Mar. 26, 2002) and2003-70135 (filed on Mar. 14, 2003) which are expressly incorporatedherein by reference in their entireties.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing a light emission mechanism of priorart organic EL device;

FIG. 2 a schematic view showing a light emission mechanism of theorganic EL device according to the present invention;

FIG. 3 is a schematic view showing a charge transfer complex formationand transfer of electrons and holes upon application of voltage in acharge generation layer having a laminated layer structure, according tothe device of the present invention;

FIG. 4 is a schematic view showing a charge transfer complex formationand transfer of electrons and holes upon application of voltage in acharge generation layer having a mixed layer structure, according to thedevice of the present invention;

FIG. 5 is a graph of the absorption spectrum obtained in a single layeror mixed layer of an arylamine compound and vanadium pentaoxide;

FIG. 6 is a graph of the absorption spectrum obtained in a single layeror mixed layer of 2-TNATA and 4F-TCNQ;

FIG. 7 is a graph of the absorption spectrum obtained in a single layeror mixed layer of α-NPD and rhenium heptaoxide;

FIG. 8 is a schematic cross-sectional view illustrating a laminationstructure of the organic EL device according to the present invention;

FIG. 9 is a schematic cross-sectional view illustrating a laminationstructure of the organic EL device produced in Reference Example 1;

FIG. 10A shows a glass substrate on which a transparent anode electrodeis coated;

FIG. 10B shows a construction of a metallic mask for organic layerformation;

FIG. 10C shows a construction of a metallic mask for cathode electrodeformation;

FIG. 10D shows schematic view illustrating a construction of the organicEL device;

FIG. 11 is a schematic cross-sectional view illustrating a laminationstructure of the organic EL device produced in Reference Example 2;

FIG. 12 is a schematic cross-sectional view illustrating a laminationstructure of the organic EL device produced in Reference Example 3;

FIG. 13 is a schematic cross-sectional view illustrating a laminationstructure of the organic EL device produced in Example 1;

FIG. 14 is a schematic cross-sectional view illustrating a laminationstructure of the organic EL device produced in Example 2;

FIG. 15 is a schematic cross-sectional view illustrating a laminationstructure of the organic EL device produced in Example 3;

FIG. 16 is a graph of the emission spectrum obtained in ReferenceExample 1, and Examples 1 and 4;

FIG. 17 is a graph of the emission spectrum obtained in ReferenceExamples 2 and 3, and Example 3;

FIG. 18 is a schematic cross-sectional view illustrating a laminationstructure of the organic EL device produced in Example 4;

FIG. 19 is a graph of the emission spectrum of three EL devices producedin Example 4;

FIG. 20 is a graph of the emission spectrum of three EL devices producedin Example 5;

FIG. 21 is a graph of the luminance-voltage curve 1 of the organic ELdevices produced in Reference Example 1, and Examples 1 and 2;

FIG. 22 is a graph of the current density-voltage curve of the ELdevices produced in Reference Example 1, and Examples 1 and 2;

FIG. 23 is a graph of the current efficiency-current density curve ofthe EL devices produced in Reference Example 1, and Examples 1 and 2;

FIG. 24 is a graph of the luminance-voltage curve of the organic ELdevices produced in Reference Examples 2 and 3, and Example 3;

FIG. 25 is a graph of the current density-voltage curve of the ELdevices produced in Reference Examples 2 and 3, and Example 3;

FIG. 26 is a graph of the current efficiency-current density, curve ofthe EL devices produced in Reference Examples 2 and 3, and Example 3;

FIG. 27 is a graph of the luminance-voltage curve of three organic ELdevices produced in Example 4;

FIG. 28 is a graph of the current density-voltage curve of three ELdevices produced in Example 4;

FIG. 29 is a graph of the current efficiency-current density curve, ofthree EL devices produced in Example 4;

FIG. 30 is a graph of the luminance-voltage curve of three organic ELdevices produced in Example 5;

FIG. 31 is a graph of the current density-voltage curve of three ELdevices produced in Example 5;

FIG. 32 is a graph of the current efficiency-current density curve ofthree EL devices produced in Example 5;

FIG. 33 is a plan view showing a device having a sandwiched structureused in the evaluation of the resistivity;

FIG. 34 is a cross-sectional view showing a device having a sandwichedstructure used in the evaluation of the resistivity;

FIG. 35 is a plan view showing a device having a coplanar arrangementstructure used in the evaluation of the resistivity;

FIG. 36 is a cross-sectional view showing a device having a coplanararrangement structure used in the evaluation of the resistivity;

FIG. 37 is a graph of the electric field-current density curve forcalculating a resistivity determined in a Test Example;

FIG. 38A is a photograph showing an emission state in the organic ELdevice described in Japanese Patent Application No. 2001-225847;

FIG. 38B is a schematic cross-sectional view illustrating a laminationstructure of the organic EL device;

FIG. 39A is a photograph showing an emission state in the organic ELdevice produced in Example 3;

FIG. 39B is a schematic cross-sectional view illustrating a laminationstructure of the organic EL device produced in Example 3; FIG. 40 is agraph showing a relation between mixed ratio (molar fraction) of theco-deposition layer of V₂O₅ and α-NPD, and resistivity;

FIG. 41 is a schematic cross-sectional view illustrating a laminationstructure of the organic EL device produced in Example 6;

FIG. 42 is a graph of the luminance-voltage curve of the organic ELdevices produced in Example 6 and a conventional device;

FIG. 43 is a graph of the current density-voltage curve of the ELdevices produced in Example 6 and a conventional device;

FIG. 44 is a graph of the current efficiency-current density curve ofthe EL devices produced in Example 6 and a conventional device; and

FIG. 45 is a graph of the luminous efficiency-luminance curve of theorganic EL devices produced in Example 6 and a conventional device.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The inventors of the present invention have conducted intensive studiesfor solving the above-mentioned problems, and have found that a solutioncan be attained if two or more laminated light-emissive units aresandwiched between a cathode electrode and an anode electrode opposed tothe cathode electrode, and each of the light-emissive units arepartitioned with a charge generation layer having a resistivity of atleast 1.0×10²Ω cm, desirably at least 1.0×10⁵Ω cm. Hereinafter, theproperty having such a resistivity is abbreviated as “electricallyinsulating”.

When a certain level of the voltage was applied between a cathode and ananode in the EL device having the above structure, only the two or morelight-emissive units located in a crossed area of the cathode and theanode can be connected as if connected in series, and thus they cansimultaneously emit light. Because of this simultaneous emission, usingthe EL device, it becomes possible to achieve a high quantum efficiencyor current efficiency which cannot be obtained in any conventional ELdevice.

As described above, according to the present invention, thelight-emissive units are connected “as if connected in series”throughout the charge generation layer. Such a series connection of thelight-emissive units means that when a certain level of the voltage wasapplied to the EL device, each charge generation layer can inject holesin a cathode direction of the device, thus playing a role for injectingelectrons in an anode direction, and as a result of injection of boththe electrons and the holes, although all the layers (light-emissiveunits and the charge generation layers) sandwiched between the anode andthe cathode are formed from an electrically insulating layer, the two ormore light-emissive units can act just as they are electricallyconnected in series as in an electrical circuit.

In other words, the organic EL device according to the present inventionresides in an organic EL device including two or more light-emissiveunits between a cathode electrode and an anode electrode opposed to thecathode electrode, each light-emissive unit having at least onelight-emissive layer, in which the light-emissive units are partitionedfrom each other by at least one charge generation layer, and the chargegeneration layer is an electrically insulating layer having aresistivity of at least more than 1.0×10² Ωcm, desirably at least1.0×10⁵ Ωcm.

Furthermore, the material used in the formation of layers constitutingeach light-emissive unit corresponds to a component sandwiched betweenthe anode and the cathode in the conventional EL devices, and thus allthe layers formed therein are electrically insulating layers having aresistivity of not less than 1.0×10² Ωcm.

The “light-emissive unit” refers to a component of the EL device havinga layer structure including at least one light-emissive layer includingan organic compound, i.e., the component of the conventional organic ELdevice from which an anode and a cathode are omitted.

Furthermore, the “charge generation layer” refers to an electricallyinsulating layer having a resistivity of not less than 1.0×10² Ωcm,desirably at least 1.0×10⁵Ω cm, and as described above, represents alayer capable of injecting an electron for an anode direction of thedevice as well as injecting a hole for a cathode direction of the deviceupon voltage being applied.

In the organic EL device of the present invention, the charge generationlayer desirably includes a laminate or a mixed layer formed from twodifferent materials. A charge transfer complex having a radical cationand a radical anion is formed upon an oxidation-reduction reactionbetween these two materials. When a voltage is applied to the EL device,a radical cation state (hole) and a radical anion state (electron) inthe charge transfer complex is transferred to a direction of the cathodeand a direction of the anode, respectively, so that a hole is injectedinto the light-emissive unit which is located on a cathode side of thecharge generation layer and is adjacent thereto, and an electron isinjected into the light-emissive unit which is located on an anode sideof the charge generation layer and is adjacent thereto.

Moreover, in the organic EL device of the present invention, the chargegeneration layer desirably includes a laminated or a mixed layer whichhas the following components:

(a) an organic compound having an ionization potential of less than 5.7eV and a hole transporting property or electron donating property; and

(b) an inorganic or organic material capable of forming a chargetransfer complex through its oxidation-reduction reaction with theorganic compound (a); and

a charge transfer complex formed upon the oxidation-reduction reactionbetween the components (a) and (b) being contained in the chargegeneration layer.

In addition, in order to easily obtain a radical cation state from anorganic compound which generally has an electron donating property, itis desirable that the organic compound has an ionization potential ofless than 5.7 eV. If the ionization potential of the organic compoundused as the component (a) is 5.7 eV or more, it is difficult to cause anoxidation-reduction between the organic compound and the compound usedas the component (b) with result of difficulty in producing a chargetransfer complex which is required in when applying the presentinvention.

More particularly, the organic compound used as the component (a) isdesirably an arylamine compound, and the arylamine compound is desirablyrepresented by the following formula (I):

wherein Ar1, Ar2 and Ar3 each independently represent an aromatichydrocarbon group which can have substituents.

tetra-p-tolyl-4,4′-diaminobiphenyl,bis(4-di-p-tolylaminophenyl)phenylmethane,N,N′-diphenyl-N,N′-di(4-methoxyphenyl)-4,4′-diaminobiphenyl,N,N,N′,N′-tetraphenyl-4,4′-diaminodiphenylether,4,4′-bis(diphenylamino)quadriphenyl,4-N,N-diphenylamino-(2-diphenylvinyl)benzene,3-methoxy-4′-N,N-diphenylaminostilbenzene, N-phenylcarbazole,1,1-bis(4-di-p-triaminophenyl)cyclohexane,1,1-bis(4-di-p-triaminophenyl)-4-phenylcyclohexane,bis(4-dimethylamino-2-methylphenyl)phenylmethane,N,N,N-tri(p-tolyl)amine,4-(di-p-tolylamino)-4′-[4-(di-p-tolylamino)styryl]stilbene,N,N,N′,N′-tetraphenyl-4,4′-diaminobiphenyl N-phenylcarbazole,4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl,4,4″-bis[N-(1-naphthyl)-N-phenylamino] p-terphenyl,4,4′-bis[N-(3-acenaphthenyl)-N-phenylamino]biphenyl,1,5-bis[N-(1-naphthyl)-N-phenylamino]naphthalene,4,4′-bis[N-(9-anthryl)-N-phenylamino]biphenyl,4,4″-bis[N-(1-anthryl)-N-phenylamino] p-terphenyl,4,4′-bis[N-(2-phenanthryl)-N-phenylamino]biphenyl,4,4′-bis[N-(8-fluoranthenyl)-N-phenylamino]biphenyl,4,4′-bis[N-(2-pyrenyl)-N-phenylamino]biphenyl,4,4′-bis[N-(2-perylenyl)-N-phenylamino]biphenyl,4,4′-bis[N-(1-coronenyl)-N-phenylamino]biphenyl,2,6-bis(di-p-tolylamino)naphthalene,2,6-bis[di-(1-naphthyl)amino]naphthalene,2,6-bis[N-(1-naphthyl)-N-(2-naphthyl)amino]naphthalene,4,4″-bis[N,N-di(2-naphthyl)amino]terphenyl, 4,4′-bis{N-phenyl-N-[4-(1-naphthyl)phenyl]amino} biphenyl,4,4′-bis[N-phenyl-N-(2-pyrenyl)amino]biphenyl,2,6-bis[N,N-di(2-naphthyl)amino]fluorene,4,4″-bis(N,N-di-p-tolylamino)terphenyl, bis(N-1-naphthyl)(N-2-naphthyl)amine, 4,4′-bis[N-(2-naphthyl)-N-phenylamino]biphenyl(α-NPD), represented by the following formula:

spiro-NPD represented by the following formula:

spiro-TAD represented by the following formula:

2-TNATA represented by the following formula:

and the like.

Furthermore, any well-known arylamine compound used in the production ofa conventional organic EL devices can be suitably used.

Furthermore, in regard to increasing a heat resistance of the devices,it is desirable that the arylamine compound used herein is an arylaminecompound having a glass transition temperature of not lower than 90° C.

Among many arylamine compounds listed above, α-NPD, spiro-NPB, spiro-TADand 2-TNATA are typical examples of a suitable arylamine compoundbecause they have a glass transition temperature of not lower than 90°C.

In the organic EL device of the present invention, if the chargegeneration layer is constructed from a laminate including two differentmaterials, one material constituting the laminate can be an organicmaterial which constitutes a hole transporting layer in thelight-emissive unit adjacent to the charge generation layer.Furthermore, in such a case, the hole transporting layer is desirablyconstructed from an arylamine compound used as the component (a).

The present invention will be further described with reference to theaccompanying drawings.

As described above, the organic EL device according to the presentinvention is characterized in that the device includes an anodeelectrode/a plurality of light-emissive units (includes at least onelight-emissive layer, principally consists of an organic material andgenerally has a laminated structure of two or more layers)/a cathodeelectrode. The plurality of light-emissive units are provided betweenthe anode and cathode electrodes, and each light-emissive unit ispartitioned with an electrically insulating charge generation layerhaving a resistivity or specific resistance of not less than 1.0×10²Ωcm, desirably not less than 1.0×10⁵ Ωcm.

As shown in FIG. 1, the prior art organic EL device has a constructionin which a single light-emissive unit is sandwiched in between theelectrodes, and an electron (e⁻) is injected from a cathode side intothe unit, while a hole (h⁺) is injected from an anode side into the unitso that the electron and the hole can be recombined inside the unit,thereby an excitation state to cause light emission.

Conversely, in the organic EL device according to the present invention,as shown in FIG. 2, a recombination of the electron and the hole can bemade within the plurality of light-emissive units, each beingpartitioned by a charge generation layer, and thus a plurality of lightemissions can be generated between the electrodes.

In the organic EL device of the present invention an electricallyinsulating material having a resistivity of not less than 1.0×10² Ωcm,desirably not less than 1.0×10⁵ Ωcm is used as a material for forming acharge generation layer. Furthermore, generally, the charge generationlayer is desirably a layer having a visible light transmittance of notless than 50%. A transmittance of less than 50% will not provide thedesired quantum efficiency (current efficiency) even if the device has aplurality of the light-emissive units because the light generated in theunits is absorbed during its transmission through the charge generationlayer.

Furthermore, both an inorganic material and an organic material can beused as a material for forming a charge generation layer, providing thematerial used has a specific resistivity described above. However, asuitable construction of the charge generation layer of the presentinvention, as described above, includes a laminate or a mixed layerformed from two different materials. Upon oxidation-reduction reactionbetween these two materials, a charge transfer complex including aradical cation and a radical anion is formed in the charge generationlayer. Since a radical cation state and a radical anion state in thecharge transfer complex are moved to a cathode direction and an anodedirection, respectively, when a voltage is applied, the chargegeneration layer can inject a hole in a light-emissive unit adjacent tothe layer on a cathode side and also can inject an electron in alight-emissive unit adjacent to the layer on an anode side.

As described above, the charge generation layer in the device of thepresent invention is desirably a laminate or a mixed layer formed froman arylamine compound such as the component (a) and a substance, such asthe component (b), which may be an inorganic substance or an organicsubstance, capable of forming a charge transfer complex upon theoxidation-reduction reaction with the arylamine compound.

FIG. 3 is a schematic view showing a charge transfer complex formationin a charge generation layer which is a laminate including theabove-described components (a) and (b), and the transfer of electronsand holes in the charge generation layer upon application of thevoltage.

Furthermore, FIG. 4 is a schematic view showing a charge transfercomplex formation and transfer of electrons and holes upon applicationof voltage in a charge generation layer which is a mixed layer includingthe above components (a) and (b).

Furthermore, whether or not the two compounds constituting the chargegeneration layer can form a charge transfer complex can be confirmed byusing a spectroscopic analysis. For example, when the two compounds areexamined, it can be confirmed that in separate use, the each compounddoes not exhibit an absorption peak in a near infrared region of thewavelength of 800 to 2,000 nm, however, if they are used as a mixedlayer, the layer can show an absorption peak in a near infrared regionof the wavelength of 800 to 2,000 nm, i.e., the confirmed absorptionpeak clearly teaches the presence (or evidence) of an electron transferbetween the two compounds.

FIG. 5 shows an absorption spectrum obtained in a sole use of each ofarylamine compounds: 2-TNATA, α-NPD, spiro-TAD and spiro-NPB, and V₂O₅(vanadium pentaoxide), and an absorption spectrum obtained in a mixedlayer of each arylamine compound and vanadium pentaoxide. As can beappreciated from the graph of FIG. 5, the arylamine compounds andvanadium pentaoxide each cannot show a peak in a near IR region of thewavelength of 800 to 2,000 nm when they are used alone, but, if they areused in the form of a mixed layer including the arylamine compound andvanadium pentaoxide, the layer can show a prominent peak in a near IRregion of the wavelength of 800 to 2,000 nm, from which a chargetransfer complex formation can be confirmed.

FIG. 6 shows an absorption spectrum of each of 2-TNATA and 4F-TCNQobtained when they are used in the form of a single layer or a mixedlayer, and FIG. 7 shows an absorption spectrum obtained in a mixed layerof α-NPD and Re2O7 (di-rhenium heptaoxide).

The inventors of the present invention could observe from the absorptionspectrums of each of the mixed layers shown in FIGS. 5 to 7 that a newand third absorption spectrum was produced in a position of the near IRregion (800 to 2,000 nm) upon the reaction caused with the electrontransfer, and the third absorption spectrum is not a simply piled upspectrum curve obtained as a result of combination of a spectrum of onesingle substance with a spectrum of another single substance. Theinventors have studied and found that a chemical reaction generated inthe mixed layer is an important factor to ensure a charge transfer uponapplication of the voltage.

When two compounds (or layers) are laminated, it is easily conceivedthat a chemical reaction can be generated in an interfacial surfacebetween the two layers. Thus, it is true that the intended and desiredproperties can be obtained in a charge generation layer when the layeris formed by lamination of the two compounds.

In the present invention, the terms “light-emissive unit”, as explainedabove, means a “component of the conventional organic EL device”excluding an anode and a cathode.

The “component of the conventional organic EL device” includes, forexample, (anode)/a light-emissive layer/(cathode), (anode)/ahole-transporting layer/a light-emissive layer/(cathode), (anode)/ahole-transporting layer/a light-emissive layer/an electron-transportinglayer/(cathode), (anode)/a hole injection layer/a hole-transportinglayer/a light-emissive layer/an electron-transporting layer/(cathode)and the like.

In the organic EL device according to the present invention, thelight-emissive units may have any laminate structure, providing thelaminate structure satisfies the requirement that each light-emissiveunit is partitioned with an electrically insulating charge generationlayer and that there are a plurality of light-emissive units.Furthermore, the materials used in the formation of a light-emissivelayer, a hole-transporting layer, a hole injection layer, anelectron-transporting layer, an electron injection layer, and the likeare not restricted to any specific material and can be any conventionalmaterial used in the formation of these layers.

Furthermore, the light-emissive or luminescent materials which may beadded to a light-emissive layer are also not restricted to a specificmaterial, and can be any well-known material which includes, forexample, a wide variety of fluorescent materials and phosphorescentmaterials.

Generally, a metal having a low work function or a metal alloy, a metaloxide, and the like, containing such a low work function metal is mainlyused as the cathode material. Specifically, the cathode materialincludes, for example, a single body of a metal, for example, analkaline metal such as Li, and the like, an alkaline earth metal such asMg, Ca, and the like, a rare earth metal such as Eu, and the like, and ametal alloy of these metals and Al, Ag, In, and the like. Furthermore,in the device construction suggested by the inventors in JapaneseLaid-open Patent Application Nos. 10-270171 and 2001-102175, in which ametal-doped organic layer is used in an interfacial surface between acathode and an organic layer, any electrically conducting material maybe used as the cathode material. In this construction, the selection ofthe cathode material is not restricted by properties such as workfunction of the selected material.

Moreover, if an organic layer adjacent to a cathode is constructed froman organic metal complex compound containing at least one of alkalinemetal ions, alkaline earth metal ions and rare metal ions using thetechnologies disclosed by the inventors in their Japanese Laid-openPatent Application Nos. 11-233262 and 2000-182774, a metal capable ofreducing a metal ion contained in the complex compound in vacuum to thecorresponding metal, for example, a thermally reducible metal such asAl, Zr, Ti, Si, and the like, or an alloy including these metals may beused as the cathode material. Among these metals, aluminum (Al) which isgenerally and widely used as a wiring material is particularly desiredas the cathode material in view of its easy vapor deposition, high lightreflectance and chemical stability.

Similarly, the anode material is not restricted to a specific material.For example, a transparent conducting material such as ITO (indium tinoxide), IZO (indium zinc oxide), and the like, can be used as the anodematerial.

Furthermore, assuming that an ITO coating is formed with a sputteringmethod using the process suggested in Japanese Patent Application No.2001-142672 to avoid damage in an organic layer, a transparentconducting material such as above described ITO and IZO may be used asthe cathode material if a metal-doped organic layer described inJapanese Laid-open Patent Application No. 10-270171 is used as anelectron injection layer in the manner described above. Accordingly, itbecomes possible to produce a transparent light-emitting device byforming both of the cathode and the anode as a transparent electrode,because the organic layer and the charge generation layer are alsotransparent. Alternatively, contrary to the structure of theabove-described general organic EL device, if an anode is formed fromany metal material and a cathode is formed as a transparent electrode,it is possible to provide a device structure in which the emitted lightcan be projected from a laminated-layers side of the device, not from asubstrate side of the device.

In addition, the order of the steps for forming layers is not restrictedto any specific order. Namely, the layer formation may not always bestarted fro an anode side of the device, and the layers may be formedfrom a cathode side of the device.

In the organic EL device of the present invention, types of the materialused in the formation of cathode and anode electrodes or the method forforming a charge injection layer adjacent to these electrodes may bebased on well-known technology widely used in the conventional ELdevices, providing that two or more light-emissive units are containedbetween the opposed cathode and anode electrodes and each light-emissiveunit is partitioned by a charge generation layer having a resistivity ofnot less than 1.0×10² Ωcm, desirably not less than 1.0×10⁵ Ωcm.

The organic EL device of the present invention having a novel devicestructure is distinguishable from the conventional organic EL devices inview of the following notably different characteristics.

Firstly, in the organic EL device of the present invention, atheoretical limitation is not applied to the quantum efficiency of thedevice, whereas in the conventional EL devices, an upper limitation ofthe quantum efficiency which is a ratio of photon (number)/sec vs.electron (number)/sec, simply determined in an external circuit, is 1(=100%) in theory. This is because an injection of hole (h⁺) shown inFIG. 2 means a generation of a radical cation as a function ofwithdrawal of electrons from a valance band (or HOMO, highest occupiedmolecular orbital) of an organic layer, and thus the electrons withdrawnfrom a valance band of the organic layer constituting a layer adjacentto the charge generation layer on a cathode side is injected into anelectron conduction band (or LUMO, lowest unoccupied molecular orbital)of an organic layer constituting a layer adjacent to the layer on ananode side, thereby producing a light-emissive excitation state. Namely,the withdrawn electrons are again utilized in the formation of alight-emissive excitation state.

Accordingly, in the organic EL device of the present invention, thequantum efficiency thereof is calculated as a sum of the quantumefficiency of each light-emissive unit partitioned with a chargegeneration layer wherein the quantum efficiency is defined as a ratio ofelectrons (apparent number) passing thorough each light-emissiveunit/sec vs. photons (number), emitted from each light-emissiveunit/sec, and therefore the quantum efficiency has no upper limit.

Namely, the organic EL device of the present invention can still beoperated as a planar and thin film-shaped light-emissive device capableof emitting light only from a crossed area of the cathode and the anodeas in the conventional organic EL devices, although it has a circuitstructure the same as that of the conventional devices in which pluralEL devices are connected in series with a metal wiring, because thepresent device has a charge generation layer having a very thin andtransparent layer structure and the charge generation layer isconstructed from an (electrically) insulating layer having a resistivitywhich is substantially the same as that of the organic layer.

Although the organic EL device of the present invention is onlyconstructed from an insulating material having a resistivity of not lessthan 1.0×10² Ωcm, desirably not less than 1.0×10⁵ Ωcm, except for theelectrodes, the organic EL device can be operated at a driving voltagewhich is a sum of the potential reduction amount (Vn) consumed in eachof the light-emissive units, i.e., V=V1+V2+ . . . +Vn, because thepresent device is consequently operated just as if the plurality (n) ofthe conventional EL devices were connected in series. Accordingly, anadvantage obtained in conventional devices, i.e., a low voltage drivingat 10 volts or less, cannot be obtained in the present device withincrease of the number (n) of the light-emissive units.

However, the organic EL device of the present invention still has someadvantages over conventional organic EL devices. In conventionaldevices, since the luminance is substantially proportional to a currentdensity, it was essentially required to apply a higher current densityto obtain an increased luminance. On the other hand, since, aspreviously mentioned, the operational life-time of the device wasinversely proportional to the current density (not to a drivingvoltage), a high luminance emission results in a shortened operationallife-time of the device.

Contrary to the drawbacks of conventional devices, in the organic ELdevice of the present invention, if it is desired to obtain an n-timesincreased luminance at a desired current density, such a increase of theluminance can be attained by increasing the number of the light-emissiveunits (each having the same construction) used between the electrodes byn-times, without increasing the current density.

In this method, the driving voltage will be also increased to a level ofn-times or more. However, it should be noted that an unexpected andimportant advantage is that an n-times increased luminance can beachieved without sacrificing the operational life-time.

Furthermore, in the organic EL device of the present invention, a layerthickness between the cathode and the anode can be naturally increasedby increasing the number of the light-emissive units used therein. Forexample, assuming that the number of the light-emissive units betweenthe electrodes is “n”, a layer thickness of the present device isincreased to about n-times of that of the conventional EL devices. Inaddition, since the number of the light-emissive units in the presentdevice is not restrictive, a layer thickness between the electrodes isalso not restrictive. In view of the fact that in-conventional ELdevices, a layer thickness between the electrodes of not exceeding 1 μm(practically, not more than 2000 Å (not more than 200 nm)) and that adriving voltage of 25 volts or less must be applied, the present ELdevice has an essentially different characteristics which cannot befound in the conventional EL devices (above mentioned Kodak Patent,Japanese Laid-open Patent Application Nos. 59-194393, 63-264692 and2-15595, U.S. Pat. Nos. 4,539,507, 4,769,292, and 4,885,211).

Namely, in the organic EL device of the present invention, there is nonecessity to define an upper limit of the layer thickness between theelectrodes, an upper limit of the driving voltage and an upper limit ofthe quantum efficiency (current efficiency).

On the other hand, in conventional organic EL devices, an increase ofthe driving voltage results only in a reduction of the power conversionefficiency (w/w). Conversely, according to the organic EL device of thepresent invention, in principle, the conversion efficiency (w/w) can bemaintained without any change, because if “n” of the light-emissiveunits are introduced between the electrodes, the light-emission startingvoltage (turn on voltage), and the like, are increased by about n-times,and accordingly the voltage for obtaining the desired luminance isincreased by about n-times, and in addition to the increase of thesevoltages, the quantum efficiency (current efficiency) can be alsoincreased by about n-times.

Moreover, the organic EL device of the present invention containing aplurality of the light-emissive units has a secondary advantage of beingable to reduce the risk of short circuiting in the device. Inconventional EL devices containing only one light-emissive unit, if anelectrical short circuit is caused between a cathode and an anode due topresence of pin-holes, etc., in the layer of the unit, the EL devicescould immediately change to a state of emitting no light. Conversely, inthe organic EL device of the present invention, since the layerthickness between the electrodes is thick, a risk of short circuitingcan be reduced, and at the same time, even if short circuiting is causedin some light-emissive units, the worst scenario result such asnon-light emission can be avoided because the remaining light-emissiveunits can still emit light. Specifically, when the EL device is designedto be driven at a constant current, a driving voltage is only reduced byan amount corresponding to the short circuited units, and the remainingnon-short circuited units can emit light normally.

In addition to the above advantages, for example, when the organic ELdevice of the present invention is applied to an EL display devicehaving a simple matrix structure, a reduction of the current densitymeans that a voltage reduction due to the wiring resistance and atemperature increase in the substrate can be largely reduced incomparison with a conventional display device. Furthermore, a higherdriving voltage between the electrodes, which sandwich thelight-emissive element portion, in comparison with the conventionaldevices means that a voltage reduction due to the wiring resistance doesnot largely cause a reduction of the luminance (the effect due thehigher driving voltage can be sufficiently understood just fromconsidering the influence of the possible potential reduction of 1 voltdue to the wiring resistance to a reduction of the luminance incomparison with an EL device capable of providing a luminance of 1,000cd/m² at 5 volts and an EL device capable of providing a luminance of1,000 cd/m² at 50 volts). This effect, in combination with anothercharacteristic of the EL device of the present invention where thedevice naturally has a low voltage reduction in the wiring portionthereof, enables to achieve a display device controllable at a constantvoltage which can not be provided using a conventional device.

Furthermore, the above-described characteristics advantageously affectother uses for obtaining an uniform light emission in a large surfacearea, in particular, for use as an illumination apparatus. Inconventional organic EL devices, since an electrode material usedtherein, especially a transparent electrode material, typically ITO,etc., has a resistivity of up to 10⁻⁴Ω cm, which is about 100Ω cm higherthan a resistivity of metal (up to 10⁻⁶Ω cm), a voltage (V) or electricfield E (V/cm) applied to the light-emissive unit reduces with anincrease of the distance from a contact point of electric power, so thatunevenness (difference of luminance) in the luminance occurs between anear portion to and a far portion from a contact point of electricpower. Conversely, according to the organic EL device of the presentinvention, since an electric current in obtaining the desired luminancecan be largely reduced in comparison with conventional EL devices, thepotential reduction can be diminished with the result that substantiallyuniform light emission can be obtained in a large surface illuminationapparatus.

Furthermore, in the formation of the charge generation layer, since thepresent invention is characterized by intentionally using a materialhaving a considerably increased resistivity (of not less than 1.0×10²Ωcm, desirably not less than 1.0×10⁵ Ωcm) than that of an ITO and otherelectrically conductive materials (about 10⁻⁴ Ωcm), a shadow mask fordefining a vapor deposition area, which is with the same as that used inthe formation of the patterned organic layer, can be used in the layerformation process of the charge generation layer, and thus the frequentchange and precise positioning of the shadow mask can be excluded fromthe production process except for the formation of the electrodes.Namely, according to the present invention, it becomes possible toachieve a remarkably increased productivity.

FIG. 8 is a schematic cross-sectional view illustrating a laminatedstructure of the organic EL device according to an embodiment of thepresent invention. A glass substrate (transparent substrate) 1 includes,laminated in sequence thereon, a transparent electrode 2 constituting ananode electrode, a light-emissive unit 3-1, a charge generation layer4-1, a light-emissive unit 3-2, a charge generation layer 4-2, . . . , acharge generation layer 4-(n−1), a light-emissive unit (3-n) wherein then=1, 2, 3, . . . , and finally a cathode electrode (metal electrode) 5.In these elements (layers), the glass substrate (transparent substrate)1, the transparent anode electrode 2, the light-emissive unit (3-n)wherein n is 1, 2, 3, . . . , and the cathode electrode 5 each is awell-known element (layer). The new feature in the EL device of thepresent invention resides in that a plurality of light-emissive units(3-n, wherein n is 1, 2, 3, . . . ) are contained between bothelectrodes and are partitioned with an electrically insulating chargegeneration layer (4-n, wherein n is 1, 2, 3, . . . ) having aresistivity of not less than 1.0×10² Ωcm.

Furthermore, in regard to organic EL devices, it is known that thecharacteristics thereof such as driving voltage, etc., can be varieddepending upon the work function; the work function being one propertyof the electrode material. Referring to the organic EL device of thepresent invention, the charge generation layer 4-n used therein is notacting as an electrode. However, since an electron is injected into adirection of the anode electrode and a hole is injected in a directionof the cathode electrode, in the formation of the above-describedcomponents of the light-emissive unit, particularly the method forforming a electron injection (transporting) layer and a hole injection(transporting) layer, both being adjacent to a charge generation layer,is essential for reducing an energy barrier in the injection of thecharge (electron and hole) into each light-emissive unit.

For example, if it is intended to inject an electron from each chargegeneration layer 4-n to a direction of the anode electrode, it isdesirable that, as is disclosed in Japanese Laid-open Patent ApplicationNos. 10-270171 and 2001-102175, an electron injection layer having amixed layer of an organic compound and a metal functioning as anelectron donating (donor) dopant, is formed as a layer adjacent to thecharge generation layer in anode side. The donor dopant desirablyincludes at least one metal selected from alkaline metals, alkalineearth metals and rare earth metals.

Furthermore, in the electron injection layer, a molar ratio of the metalas the donor dopant is desirably in the range of 0.1 to 10 with respectto the organic compound. A molar ratio of less than 0.1 results in areduction of the doping effect because a concentration of the moleculereduced with the dopant (hereinafter, referred to as a “reducedmolecule”) is reduced excessively. A molar ratio above 10 also resultsin a reduction of the doping effects because a concentration of thedopant in the layer is significantly increased in comparison withconcentration of the organic compound, thus causing an excessivereduction of the reduced molecule in the layer.

The application of the above-described electron injectionlayer-containing structure to a light-emissive unit of the organic ELdevice achieves an energy barrier-free electron injection to each of thelight-emissive units regardless of the work function of the materialconstituting a charge generation layer.

Furthermore, the light-emissive unit may have a structure in which anelectron injection layer including a metal selected from alkalinemetals, alkaline earth metals and rare earth metals, and having a layerthickness of up to 5 nm (desirably 0.2 to 5 nm) is disposed as a layeradjacent to the charge generation layer on an anode side. A layerthickness of above 5 nm is not desirable because it reduces a lighttransmittance, and at the same time, makes the device unstable becausethe content of the metal which has a high reactivity and is unstable inair is excessively increased in the layer. Moreover, in this metal layerhaving a layer thickness of up to 5 nm, it is considered that asubstantial amount of the metal layer can be diffused into an organiclayer to result in a layer having a composition which is substantiallythe same as that of the above-described metal doping layer. Theresulting layer at least has no form of the metal layer having anelectrical conductivity.

For example, if the electron is injected from each charge generationlayer 4-n in the anode direction, it is also desirable that the electroninjection layer, which is disclosed in Japanese Laid-open PatentApplication Nos. 11-233262 and 2000-182774 (corresponding U.S. Pat. No.6,396,209) (J. Endo, T. Matsumoto, and J. Kido, Jpn. J. Appl. Phys. Vol.41 (2002) pp. L800-L803), is provided on the anode side of the chargegeneration layer. The electron injection layer of this type is explainedas an “in-situ reaction generating layer” which is generated bydepositing a thermally reducible metal such as aluminum on a compoundcontaining an alkaline metal ion, an alkaline earth metal ion and a rareearth metal ion to reduce the metal ions into a metal condition. In thedevice of the present invention, it is desirable to supply the very thinthermally reducible metal on the compound by a minimum amount requiredfor the reduction reaction. If the metal ion in the compound is reduced,the supplied thermally reducible metal itself is oxidized to be ainsulative compound having a resistivity not less than 1.0×10² Ωcm. Thevery thin thermally reducible metal has a layer thickness not more than10 nm. If the layer thickness of the thermally reducible metal is morethan 10 nm, a metal atom, which does not contribute to the reductionreaction, remains so that the transparency and insulation property arelost.

In addition to an organic metal complex compound described in the abovementioned patent document (Japanese Laid-open Patent Application Nos.11-233262 and 2000-182774), an inorganic compound can be used as thecompound including the alkaline metal ion, alkaline earth metal ion andrare earth metal ion, which are used for the above mentioned “in-situreaction generating layer”. An oxide and halide including the alkalinemetal ion, alkaline earth metal ion and rare earth metal ion can be usedas the compound for the in-situ reaction generating layer, and further,any inorganic compound including the alkaline metal ion, alkaline earthmetal ion and rare earth metal ion can be used as the compound.

Furthermore, it is also desirable to use different types of electroninjection (transporting) layers in above mentioned Japanese Laid-openPatent Application Nos. 10-270171, 2001-102175, 11-233262 and2000-182774 (corresponding to U.S. Pat. No. 6,396,209) in a superposedcondition. The metal doping layer in the Japanese Laid-open PatentApplication No. 10-270171 or 2001-102175 is desirably deposited on theorganic layer (including light-emissive layer), by a predeterminedthickness, as a low resistance electron transporting layer, then thein-situ reaction generating layer described in the Japanese Laid-openPatent Application Nos. 11-233262 and 2000-182774 is superposed on themetal doping layer. As mentioned above, a technical idea in which anelectron injection layer contacting the cathode electrode of theconventional electroluminescent device is formed by using superposeddifferent types of electron injection (transporting) layers is describedin the Japanese Patent Application No. 2002-273656 by the inventors ofthe present invention.

In this case, the in-situ reaction generating layer contacts the chargegeneration layer on an anode side. According to the present invention,an interaction between a material used for the charge generation layerand a reactive metal such as alkaline metal can be avoided. As a result,it is found that such method is a desirable for forming an electroninjection layer on a point that the electron injection barrier from thecharge generation layer to the light emissive unit can be lowered.

Furthermore, for example in the injection of holes from each chargegeneration layer 4-n to a direction of the cathode electrode, a holeinjection layer, suggested by the inventors in Japanese Laid-open PatentApplication Nos. 11-251067 and 2001-244079, which contains a dopedelectron-accepting compound (Lewis acid compound) having a property ofoxidizing an organic compound in terms of Lewis acid chemistry may beformed as a layer adjacent to the charge generation layer in a cathodeside. Regardless of the work function of the material constituting thecharge generation layer 4-n, hole injection in the absence of an energybarrier can be achieved.

Moreover, a layer of the electron-accepting compound (Lewis acidcompound) which is very thin and thus ensures a transparency may beformed as a hole injection layer. In this method, a layer thickness ofthe hole injection layer is desirably 30 nm or less, more desirably inthe range of 0.5 to 30 nm. The layer thickness above 30 nm causes areduction of the light transmittance, and at the same time, makes thedevice unstable because a content of the Lewis acid compound which has ahigh reactiveness and is unstable in air is excessively included in thelayer.

The electron-accepting compound (Lewis acid compound) used herein is notrestricted to a specific compound. For example, electron-acceptingcompound includes an inorganic compound such as ferric chloride, ferricbromide, ferric iodide, aluminum chloride, aluminum bromide, aluminumiodide, gallium chloride, gallium bromide, gallium iodide, indiumchloride, indium bromide, indium iodide, antimony pentachloride, arsenicpentafluoride, boron trifluoride, and the like, and an organic compoundsuch as DDQ (dicyanodichloroquinone), TNF (trinitrofluorenone), TCNQ(tetracyanoquinodimethane), 4F-TCNQ(tetrafluoro-tetracyanoquinodimethane), and the like.

In the hole injection layer, a molar ratio of the organic compound andthe electron-accepting compound (dopant compound) is desirably in therange of 0.01 to 10 with respect to the organic compound. A molar ratioof less than 0.01 results in a reduction of the doping effects because aconcentration of the molecule oxidized with the dopant (hereinafter,referred also to an “oxidized molecule”) is excessively reduced. A molarratio above 10 also results in a reduction of the doping effects becausea concentration of the dopant in the layer is remarkably increased incomparison with concentration of the organic compound, thus causing anexcessive reduction of a concentration of the oxidized molecule in thelayer.

Further, if the material forming a charge generation layer has a workfunction of not less than 4.5 eV, it may be sometimes possible to injectholes to each light-emissive unit without specially using anelectron-accepting compound (Lewis acid compound).

Conversely, as shown in Example 2 described hereinafter, the Lewis acidcompound itself may sometimes act as a component of the chargegeneration layer.

In the light-emissive units used in the present invention, the layerswhich are formed in direct contact with the cathode or anode may havethe same composition as that of the layer adjacent to the chargegeneration layer on an anode side or the layer adjacent to the chargegeneration layer on a cathode side, respectively, or the electroninjection layer and the hole injection layer each may have some othercompositions. Of course, the electron injection layer and the holeinjection layer used in the conventional EL devices may be suitablyused.

In comparison with conventional organic EL devices, the amount of timeit takes for the layer to be formed in the production of the organic ELdevice of the present invention is necessarily longer. Furthermore,since the present method is characterized in that the substantially sameprocesses are repeatedly-carried out, conventional batch system-basedvapor deposition apparatuses which are currently widely used for layerformation require an excessively long processing time. Moreover, anincrease of the production costs is of concern because a large amount ofexpensive organic materials must be used, compared to conventionalorganic EL devices.

In such a case, it is suggested by the inventors in Japanese PatentApplication No. 2001-153367 to use an in line-line system-basedcontinuous layer formation apparatus. Using this apparatus, the timerequired for layer formation can be largely shortened and the efficiencyof materials use can be increased so as to approach 100%.

Furthermore, in the formation of the organic layer, the chargegeneration layer and the electrode layer which constitute the organic ELdevice of the present invention, any well-known deposition method whichis conventionally used such as a resistive heating vapor depositionmethod, an electron beam vapor deposition method, a laser beam vapordeposition method, a sputtering method, and the like, can be used.

In particular, when an inorganic substance or compound such as metaloxide is used as an element for forming a charge generation layer, avapor deposition method must be carried out with care, because there isa tendency that a deposited layer may have a composition which isoutside of the desired stoichiometric composition due to separation,etc., of oxygen atoms from the compound.

Furthermore, when an inorganic substance or compound is deposited usinga sputtering method, it is important to use a method in which asubstrate having the formed organic layer is disposed separately fromthe plasma generated during the deposition process to thereby avoiddamage of the organic layer. At the same time, it is also important thatthe molecules of the sputtered inorganic compounds are softly depositedon the organic layer with a kinetic energy up to a predetermined levelin order to reduce damage in the device.

For example, the facing target sputtering apparatus in which a pair ofopposed targets arranged separately from each other at a certaindistance have a reflection electrode for reflecting electrons against afront peripheral portion of each of the targets, and a magnetic fieldgenerating device which is included to form a parallel magnetic fieldhaving a portion parallel to a surface of the target in the vicinity ofthe peripheral portion of each target (see, Japanese Patent ApplicationNo. 2001-142672) can be suitably used in the formation of the chargegeneration layer of the present invention, too.

In addition, all the layers to be formed on a substrate can be formed bythe vapor deposition method in which all layers are formed on asubstrate by heating a vaporizable material in a vacuum to deposit avaporized or sublimed material on the substrate, and includestransporting a substrate in a direction of a planar surface thereof, adeposition area being open in a lower surface of the substrate;providing a container, in a lower position of the transportingsubstrate, including a vaporizable material having a deposition widthwhich can cover the deposition area extending in a directionperpendicular to the transportation direction of the substrate; andheating the container, thereby vaporizing or subliming and thusdepositing the vaporizable material in the container (Japanese PatentApplication No. 2001-153367).

Furthermore, contrary to conventional EL devices, using the organic ELdevice of the present invention, the highest light emission efficiencycan be obtained when an optical path length from light-emissive site tolight-reflective electrode is almost an odd-numbered-times greater thana quarter wavelength of light, i.e., λ×(2n−1)/4 wherein n is 1, 2, 3, .. . , since an important feature in the present invention is that two ormore light-emissive site are provided at intervals.

In conventional EL devices, a structure is adopted wherein an opticalpath length from light-emissive site to light-reflective electrode isadjusted to approximately an odd-numbered times a quarter wavelength oflight. In such devices, even if the organic layer is formed at a largerthickness above the quarter wavelength of light, the result is only anundesirable increase of the driving voltage.

However, as disclosed in above mentioned Japanese Laid-open PatentApplication No. 2001-102175, if a Combination of the electrontransporting organic compound and the alkaline metal (both constitutingan electron injection layer adjacent to a light-reflective cathode) areappropriately selected, it becomes possible to inhibit an increase ofthe driving voltage at a larger layer thickness of about 1 μm, and acolor hue (namely, a profile of the emission spectrum) can be largelychanged because an interference effect can be remarkably increased withan increase of the layer thickness.

For example, assuming that an optical path length of the electroninjection layer is adjusted to be approximately an odd-numbered times aquarter wavelength of light, i.e., λ×(2n−1)/4 wherein n is 1, 2, 3, . .. , a profile of the resulting emission spectrum is narrowed by anincrease of n. On the other hand, if an optical path length of theelectron injection layer is adjusted to be approximately aneven-numbered times a quarter wavelength of light, i.e., λ×(2n)/4wherein n is 1, 2, 3, . . . , there arises a notable interference effectwith an increase of n, with the result that the emission efficiency islargely deteriorated because an emission in the original light-emittingpeak is offset with the notable interference effect.

Accordingly, when the organic EL device has the resultant structure inwhich n is large and a plurality of light-emissive site are contained asin the EL device of the present invention, it is essential to exactlycontrol the layer thickness from each light-emissive site to alight-reflective electrode.

To be free from such troublesome fine adjustment of layer thickness, itis desirable to construct the cathode electrode, which was lightreflective electrode conventionally when the anode electrode is atransparent electrode, a non-reflective black electrode, or to constructat least one layer existed in the cathode electrode direction so as tofunction as a light absorbing layer. Accordingly, problems with lightinterference can be avoided.

Conversely, if the anode electrode is the light reflective electrode, itis desirable that the anode electrode itself or at least one layerexisting in the anode electrode direction have a light absorbingfunction.

If a light diffuse reflection surface is provided on one of theelectrodes when the other electrode is the transparent electrode,problems with light interference can be avoided in theory.

Furthermore, as is shown in the appended examples, another feature ofthe present invention is that the light-emissive units each havedifferent emission colors so that a desired mixed (superimposed) coloremission can be obtained. In this case, it is also necessary to optimizethe optical path length from light-emissive site to light-reflectiveelectrode in the manner described above. The necessity for theoptimization of the layer thickness will depend on the emission color ineach light-emissive unit.

EXAMPLES

The present invention will be further described with reference to theexamples below. Note, however, that the present invention is notrestricted to these examples.

In the following examples, the vapor deposition of the organic compoundand the metal, as well as formation of the charge generation layer, wascarried out by using a vapor deposition apparatus commercially availablefrom VIEETECH JAPAN. The control of the deposition rate of the vapordeposition material and of the thickness of the deposited layers iscarried out by using a thickness monitor, provided with a quartzoscillator and attached to the vapor deposition apparatus, “CRTM-8000”commercially available from ULVAC. Furthermore, to determine an actuallayer thickness after the layer formation, a stylus step meter “P10”commercially available from Tencor, Co., was used. Furthermore, thecharacteristics of the organic EL device were evaluated with the sourcemeter “2400”, commercially available from KEITHLEY, and the luminancemeter “BM-8”, commercially available from TOPCON. A DC voltage wasstepwise applied at an increasing rate of 0.2 volts per 2 seconds to theEL device having an ITO anode and an aluminum (Al) cathode, and theluminance and the electric current were determined after a lapse of onesecond from the completion of each increase of the voltage. The ELspectrum was determined by using the optical multi-channel analyzer,“PMA-11” commercially available from HAMAMATSU PHOTONICS, driven at aconstant electric current.

Reference Example 1

(Example for the Production of the Conventional Organic EL Device-GreenLight-Emitting Device)

The conventional organic EL device having a laminate structure shown inFIG. 9 was produced as follows.

A glass substrate 1 used herein includes, coated in the predeterminedpattern on a surface thereof, a transparent anode electrode 2 includingan ITO (indium-tin oxide, sputtered product commercially available fromASAHI GLASS, or ion plating product commercially available from NipponSheet Glass Co., Ltd.) having a sheet resistance of about 20Ω/□ (Ω/sq.)(see, FIG. 10A). Alpha(α)-NPD having a hole transporting property wasdeposited, through a metal mask (shadow mask) 40 for organic layerformation (see FIG. 10B), onto the ITO-coated glass substrate 1 undervacuum of about 10⁻⁶ Torr and at a deposition rate of about 2 Å/sec toform a hole transportation layer 6 having a thickness of about 700 Å.

An organic-metal complex of tris(8-quinolinolato) aluminum (hereinafter,briefly referred to as “Alq”) is represented by the following formula:

and a coumarin derivative which is a green light-emissive fluorescentdye, “C545T” (trade name) commercially available from KODAK, weredeposited onto the hole transportation layer 6 under vacuum vapordeposition conditions to form a light-emissive layer 7 having athickness of about 400 Å. Each deposition rate was adjusted so that theresulting light-emissive layer 7 contains a fluorescent dye in aconcentration of about 1% by weight.

Thereafter, bathocuproine represented by the following formula:

and metal cesium (Cs) in a molar ratio of about 1:1 were co-depositedunder vacuum vapor deposition conditions to form a metal (Cs)-dopedelectron injection layer 8 having a thickness of about 200 Å on thelight-emissive layer 7. Each deposition rate was adjusted to obtain themolar ratio of about 1:1.

Finally, aluminum (Al) was deposited through a metal mask (shadow mask)41 for cathode layer formation (see, FIG. 10C) at a deposition rate ofabout 10 Å/sec onto the electron injection layer 8 to form a cathodeelectrode 5 having a thickness of about 1,000 Å. An organic EL devicehaving a square light-emissive area of 0.2 cm (length) by 0.2 cm (width)was thus obtained (see, FIG. 10D).

FIG. 16 shows an emission spectrum of the resultant organic EL device.

In this organic EL device, a DC voltage was applied between the anodeelectrode (ITO) and the cathode electrode (Al), and the characteristicsof the green light emitted from the light-emissive layer (co-depositedlayer of Alq and C545T) 7 were measured to obtain the results plotted inFIGS. 21, 22 and 23.

In FIGS. 21, 22 and 23, the circle symbols (◯) designate the luminance(cd/m²)−voltage (v) characteristic curve, a graph of current density(mA/cm²)−voltage (v) characteristic curve and a graph of currentefficiency (cd/A)−current density (mA/cm²) characteristic curve,respectively, of the EL device of Reference Example 1.

In the EL device of Reference Example 1, a voltage at which the emissionwas started was 2.2 volts.

Reference Example 2

(Example for the Production of the Conventional Organic EL Device-BlueLight-Emitting Device)

A conventional organic EL device having a laminate structure shown inFIG. 11 was produced in accordance with the manner similar to ReferenceExample 1 as follows.

A glass substrate 1 used herein includes, coated in the predeterminedpattern on a surface thereof, a transparent anode electrode 2 includingan ITO (indium-tin oxide, sputtered product commercially available fromASAHI GLASS) having a sheet resistance of about 20Ω/□ (see, FIG. 10A).Spiro-NPB having a hole transporting property was deposited, through ametal mask 40 for organic layer formation (see, FIG. 10B), onto theITO-coated glass substrate 1 under vacuum of about 10⁻⁶ Torr and at adeposition rate of about 2 Å/sec to form a hole transportation layer 9having a thickness of about 800 Å.

Spiro-DPVBi represented by the following formula:

was deposited onto the hole transportation layer 9 under vacuum vapordeposition conditions to form a light-emissive layer 10 having athickness of about 400 Å.

Thereafter, as in Reference Example 1, bathocuproine and metal cesium(Cs) in a molar ratio of about 1:1 were co-deposited under thecontrolled vacuum vapor deposition conditions to form a metal (Cs)-dopedelectron injection layer 11 having a thickness of about 200 Å on thelight-emissive layer 10.

Finally, aluminum (Al) was deposited through a metal mask 41 for cathodelayer formation (see, FIG. 10C) at a deposition rate of about 10 Å/seconto the electron injection layer 11 to form a cathode electrode 5having a thickness of about 1,000 Å. An organic EL device having asquare light-emissive area of 0.2 cm (length) by 0.2 cm (width) was thusobtained (see, FIG. 10D). In FIG. 17, an emission spectrum of theresultant organic EL device (Reference Example 2) is shown as a dottedline.

In this organic EL device, a DC voltage was applied to between the anodeelectrode (ITO) and the cathode electrode (Al), and the characteristicsof the blue light emitted from the light-emissive layer (spiro-DPVBi) 10were measured to obtain the results plotted in FIGS. 24, 25 and 26.

In FIGS. 24, 25 and 26, white circle symbols (◯) represent a graph ofluminance (cd/m²)−voltage (v) characteristic curve, a graph of currentdensity (mA/cm²)−voltage (v) characteristic curve and a graph of currentefficiency (cd/A)−current density (mA/cm²) characteristic curve,respectively, of the EL device of Reference Example 2.

In the EL device of Reference Example 2, the voltage at which theemission was started was 2.6 volts.

Reference Example 3

(Example for the Production of the Conventional Organic EL Device-RedLight-Emitting Device)

A conventional organic EL device having a laminate structure shown inFIG. 12 was produced in a manner similar to Reference Example 1 asfollows.

A glass substrate 1 used herein includes, coated in the predeterminedpattern on a surface thereof, a transparent anode electrode 2 includingan ITO (indium-tin oxide, sputtered product commercially available fromASAHI GLASS) having a sheet resistance of about 20Ω/□ (see, FIG. 10A).α-NPD having a hole transporting property was deposited, through a metalmask 40 for organic layer formation (see, FIG. 10B), onto the ITO-coatedglass substrate 1 under vacuum of about 10⁻⁶ Torr and at a depositionrate of about 2 Å/sec to form a hole transportation layer 12 having athickness of about 700 Å.

Alq and a red light-emissive fluorescent dye, “DCJTB” (trade name)commercially available from KODAK, were deposited onto the holetransportation layer 12 under the vacuum vapor deposition conditions toform a light-emissive layer 13 having a thickness of about 400 Å. Eachdeposition rate was adjusted so that the resulting light-emissive layer13 contains the fluorescent dye in a concentration of about 1% byweight.

Thereafter, as in Reference Example 1, bathocuproine and metal cesium(Cs) in a molar ratio of about 1:1 were co-deposited under thecontrolled vacuum vapor deposition conditions to form a metal (Cs)-dopedelectron injection layer 14 having a thickness of about 200 Å on thelight-emissive layer 13.

Finally, aluminum (Al) was deposited through a metal mask 41 for cathodelayer formation (see, FIG. 10C) at a deposition rate of about 10 Å/seconto the electron injection layer 11 to form a cathode electrode 5having a thickness of about 1,000 Å. An organic EL device having asquare light-emissive area of 0.2 cm (length) by 0.2 cm (width) was thusobtained (see, FIG. 10D).

In FIG. 17, an emission spectrum of the resultant organic EL device(Reference Example 3) is shown with a chain line.

In this organic EL device, a DC voltage was applied to between the anodeelectrode (ITO) and the cathode electrode (Al), and the characteristicsof the red light emitted from the light-emissive layer (co-depositedlayer of Alq and DCJTB) 13 were measured to obtain the results plottedin FIGS. 24, 25 and 26.

In FIGS. 24, 25 and 26, plus symbols (+) represent a graph of luminance(cd/m²)−voltage (v) characteristic curve, a graph of current density(mA/cm²)−voltage (v) characteristic curve and a graph of currentefficiency (cd/A)−current density (mA/cm²) characteristic curve,respectively, of the EL device (Reference Example 3).

In the EL device of Reference Example 3, a voltage at which the emissionwas started was 2.2 volts.

Example 1

(Example for the Production of the Organic EL Device Having a ChargeGeneration Layer Including V₂O₅, Vanadium Pentaoxide)

The organic EL device according to the present invention having alaminate structure shown in FIG. 13 was produced as follows.

In accordance with the manner and order described in Reference Example1, a light-emissive unit 3-1 was deposited through a metal mask 40 fororganic layer formation (see, FIG. 10B) on an ITO pattern-coated glasssubstrate 1 shown in FIG. 10A. Namely, a 600 Å-thick α-NPD, a 400Å-thick layer including Alq:C545T=100:1 (weight ratio), and a 200Å-thick mixed layer including bathocuproine and metal cesium (Cs) weresequentially deposited.

Subsequently, V₂O₅ (vanadium pentaoxide) was deposited onto themetal-doped layer at a deposition rate of about 2 Å/sec to form a chargegeneration layer 4-1 having a thickness of about 100 Å. The formation ofthe charge generation layer 4-1 was also carried out in the presence ofthe metal mask 40 for organic layer formation (see, FIG. 10B).

Thereafter, while the metal mask 40 for organic layer formation (FIG.10B) is still on the glass substrate 1, the above-described step wasagain repeated to form a light-emissive unit 3-2. Namely, a 600 Å-thickα-NPD, a 400 Å-thick layer including Alq:C545T=100:1 (weight ratio), anda 200 Å-thick mixed layer including bathocuproine and a metal cesium(Cs) were sequentially deposited.

Finally, aluminum (Al) was deposited through a metal mask 41 for cathodelayer formation (see, FIG. 10C) at a deposition rate of about 10 Å/seconto the light-emissive unit 3-2 to form a cathode electrode 5 having athickness of about 1,000 Å. An organic EL device having a squarelight-emissive area of 0.2 cm (length) by 0.2 cm (width) was thusobtained (see, FIG. 10D).

In this organic EL device, a DC voltage was applied to between the anodeelectrode (ITO) and the cathode electrode (Al), and the characteristicsof the green light emitted from the light-emissive layer (co-depositedlayer of Alq and C545T) were measured to obtain the results plotted inFIGS. 21, 22 and 23. In FIGS. 21, 22 and 23, white square symbols (□)represent a graph of luminance (cd/m²)−voltage (v) characteristic curve,a graph of current density (mA/cm²)−voltage (v) characteristic curve,and a graph of current efficiency (cd/A)−current density (mA/cm²)characteristic curve, respectively, of the EL device of Example 1.

In this EL device, a voltage at which the emission was started was 4.4volts, i.e., exactly 2 times of the voltage observed in ReferenceExample 1.

As can appreciated from the above results, the organic EL device whichincludes two light-emissive units, each partitioned with a chargegeneration layer, achieves an increased maximum current efficiency (andthus the quantum efficiency) by about 2 times in comparison with theorganic EL device of Reference Example 1.

In the EL device of Example 1, it is considered that anoxidation-reduction reaction was induced between the molecules ofvanadium pentaoxide (V₂O₅) and α-NPD, an arylamine compound which actsas a hole transporting molecule, to form a charge transfer complex (V₂O₅⁻+α−NPD⁺). Namely, an interfacial surface between the vanadiumpentaoxide (V₂O₅) layer and the α-NPD layer acts as a charge generationlayer.

In FIG. 16, an emission spectrum of the resultant organic EL device isshown by a chain line. Referring to the plotted emission spectrum, it isobserved that the spectrum is substantially the same as that ofReference Example 1, however, a full width at half maximum of thespectrum is slightly narrowed in comparison with that of ReferenceExample 1. It therefore can be concluded that this is due to thegenerated interference effect. Namely, an interference effect wasgenerated in the two light-emissive units because a light emitted fromthe firstly formed light-emissive unit 3-1 was reflected on the cathode,and the reflected light had a phase which substantially corresponds to aphase of the light directly projected in the direction of the substratefrom the emissive site.

Example 2

(Example for the Production of the Organic EL Device Having a ChargeGeneration Layer Consisting Only of an Organic Compound)

The organic EL device according to the present invention having alaminate structure shown in FIG. 14 was produced as follows.

In accordance with the manner which is substantially the same as thatdescribed in Reference Example 1, a light-emissive unit 3-1 wasdeposited through a metal mask 40 for organic layer formation (see, FIG.10B) on an ITO pattern-coated glass substrate 1 shown in FIG. 10A.Namely, a 700 Å-thick α-NPD, a 400 Å-thick layer includingAlq:C545T=100:1 (weight ratio), and a 200 Å-thick mixed layer includingbathocuproine and metal cesium (Cs) were sequentially deposited.

4F-TCNQ represented by the following formula:

was deposited onto the metal-doped layer at a deposition rate of about 1Å/sec to form a charge generation layer 4-1 having a thickness of about20 Å. 2-TNATA (product of BANDO CHEMICAL) was deposited onto the chargegeneration layer 4-1 at a deposition rate of about 1 Å/sec to obtain alayer thickness of about 50 Å.

The formation of the charge generation layer 4-1 was also carried out inthe presence of the metal mask 40 for organic layer formation (see, FIG.10B).

Thereafter, while the metal mask 40 for organic layer formation (FIG.10B) is still on the glass substrate 1, the above-described step wasagain repeated to form a light-emissive unit 3-2. Namely, a 700 Å-thickα-NPD, a 400 Å-thick layer including Alq:C545T=100:1 (weight ratio), anda 200 Å-thick mixed layer including bathocuproine and a metal cesium(Cs) were sequentially deposited.

Finally, aluminum (Al) was deposited through a metal mask 41 for cathodelayer formation (see, FIG. 10C) at a deposition rate of about 10 Å/seconto the light-emissive unit 3-2 to form a cathode electrode 5 having athickness of about 1,000 Å. The organic EL device having a squarelight-emissive area of 0.2 cm (length) by 0.2 cm (width) was thusobtained (see, FIG. 10D).

In this organic EL device, a DC voltage was applied between the anodeelectrode (ITO) and the cathode electrode (Al), and the characteristicsof the green light emitted from the light-emissive layer (co-depositedlayer of Alq and C545T) were measured to obtain the results plotted inFIGS. 21, 22 and 23.

In FIGS. 21, 22 and 23, the plus symbols (+) represent a graph ofluminance (cd/m²)−voltage (v) characteristic curve, a graph of currentdensity (mA/cm²)−voltage (v) characteristic curve and a graph of currentefficiency (cd/A)−current density (mA/cm²) characteristic curve,respectively, of the EL device of Example 2.

In the EL device of Example 2, it is considered that a charge transfercomplex (4F-TCNQ−+2-TNATA+) was formed between the two organicmolecules, i.e., 4F-TCNQ which is a Lewis acid and 2-TNATA which is ahole transporting arylamine molecule. Namely, an interfacial surfacebetween the 4F-TCNQ layer and the 2-TNATA layer acts as a chargegeneration layer.

Furthermore, in this EL device, it was observed that the currentefficiency was gradually reduced from a luminance of about 30 cd/m²(current density=0.12 mA/cm²), but the maximum current efficiency ofabout 25.6 cd/A was obtained at a current density range of up to about0.1 mA/cm². The maximum current efficiency of about 25.6 cd/A is a valuewhich could not be obtained in the conventional organic EL deviceshaving only one light-emissive unit, and proves that the chargegeneration layer can be formed by using only an organic compound.

Example 3

(Example for the Production of the Organic EL Device Having TwoLight-Emissive Units Having Different Emission Spectrums)

The organic EL device according to the present invention having alaminate structure shown in FIG. 15 was produced as follows.

As in Reference Example 1, a glass substrate 1 includes, coated in apredetermined pattern on a surface thereof, a transparent anodeelectrode 2 including an ITO (indium-tin oxide, sputtered productcommercially available from ASAHI GLASS) having a sheet resistance ofabout 20Ω/□ (see, FIG. 10A). On the ITO pattern-coated glass substrate1, in the same order as in Reference Example 2, spiro-NPB having a holetransporting property (product of COVION) was deposited through a metalmask 40 for organic layer formation (see, FIG. 10B) onto the ITO-coatedglass substrate 1 under vacuum of about 10⁻⁶ Torr and at a depositionrate of about 2 Å/sec to form a hole transportation layer of thelight-emissive unit 3-1 having a thickness of about 800 Å.

Subsequently, spiro-DPVBi (product of COVION) was deposited onto thehole transportation layer at a deposition rate of about 2 Å/sec to forma blue light-emissive layer of the light-emissive unit 3-1 having athickness of about 400 Å, followed by depositing a 200 Å-thick mixedlayer including bathocuproine and a metal cesium (Cs).

Subsequently, as in Example 1, V₂O₅ (vanadium pentaoxide) was depositedonto the mixed layer including bathocuproine and Cs at a deposition rateof about 2 Å/sec to form a charge generation layer 4-1 having athickness of about 100 Å. The formation of the charge generation layer4-1 was also carried out in the presence of the metal mask 40 fororganic layer formation (see, FIG. 10B).

Thereafter, as in Reference Example 3, α-NPD was deposited at a layerthickness of about 700 Å to form a hole transportation layer of thelight-emissive unit 3-2. Subsequently, Alq and a red light-emissivefluorescent dye, “DCJTB” (KODAK), were deposited onto the holetransportation layer to form a red light-emissive layer having athickness of about 400 Å. Each deposition rate was adjusted so that theresulting red light-emissive layer contains the fluorescent dye in aconcentration of about 1% by weight. Subsequently, as in the mannerdescribed above, a 200 Å-thick mixed layer including bathocuproine andCs was deposited.

Finally, aluminum (Al) was deposited through a metal mask 41 for cathodelayer formation (see, FIG. 10C) at a deposition rate of about 10 Å/seconto the mixed layer of bathocuproine and Cs to form a cathode electrode5 having a thickness of about 1,000 Å. The organic EL device having asquare light-emissive area of 0.2 cm (length) by 0.2 cm (width) was thusobtained (see, FIG. 10D).

In FIG. 17, an emission spectrum of the organic EL device obtained inExample 3 is shown with a solid line. In this organic EL device, a DCvoltage was applied to between the anode electrode (ITO) and the cathodeelectrode (Al). As a result, a light emission of the mixed color of blueand red (pink-colored emission) could be obtained from the twolight-emissive layers. FIG. 39A is a photograph showing an emissionstate in this device (FIG. 39B).

Subsequently, the characteristics of the device were measured to obtainthe results plotted in FIGS. 24, 25 and 26. In these drawings, whitesquare symbols (□) represent a graph of luminance (cd/m²)−voltage (v)characteristic curve, a graph of current density (mA/cm²)−voltage (v)characteristic curve, and a graph of current efficiency (cd/A) currentdensity (mA/cm²) characteristic curve, respectively, of the EL device ofExample 3.

In the EL device of Example 3, a voltage at which the emission wasstarted was about 4.8 volts. Namely, the starting voltage of about 4.8volts is a sum of the starting voltage (2.6 volts) of the device ofReference Example 2 and the starting voltage (2.2 volts) of the deviceof Reference Example 3.

Furthermore, in the EL device of Example 3, as in Example 1, it isconsidered that an oxidation-reduction reaction was induced between themolecules of vanadium pentaoxide (V₂O₅) and α-NPD, an arylamine compoundwhich acts as a hole transporting molecule, to form a charge transfercomplex (V₂O₅ ⁻+α-NPD⁺). Namely, an interfacial surface between thevanadium pentaoxide (V₂O₅) layer and the α-NPD layer acts as a chargegeneration layer.

Example 4

(Example for the Production of the Organic EL Device Having ThreeLight-Emissive Units; Experiments for Optimizing the Optical PathLength, a Distance from Each Light-Emissive Site to a ReflectiveCathode.)

The organic EL device according to the present invention having alaminate structure shown in FIG. 18 was produced as follows.

Three sheets of the ITO pattern-coated glass substrate 1 were provided.In accordance with the manner and order described in Reference Example1, a light-emissive unit 3-1 was deposited through a metal mask 40 fororganic layer formation (see, FIG. 10B) on the ITO pattern-coated glasssubstrate 1 shown in FIG. 10A. Namely, a 700 Å-thick α-NPD, a 600Å-thick layer including Alq:C545T=100:1 (weight ratio), and a 100Å-thick mixed layer including bathocuproine and a metal cesium (Cs) weresequentially deposited on each ITO pattern-coated glass substrate 1.

Subsequently, V₂O₅ (vanadium pentaoxide) was deposited onto themetal-doped layer at a deposition rate of about 2 Å/sec to form a chargegeneration layer 4-1 having a thickness of about 300 Å. The formation ofthe charge generation layer 4-1 was also carried out in the presence ofthe metal mask 40 for organic layer formation (see, FIG. 10B).

Thereafter, while the metal mask 40 for organic layer formation (FIG.10B) is still on the glass substrate 1, the above-described step wasagain repeated to form a light-emissive unit 3-2 and a light-emissivelayer 3-3. Note, in this example, that to ascertain the optimumconditions for an optical path length from each light-emissive site to areflective cathode, a layer thickness of the hole transportation layerincluding α-NPD was varied with intention to obtain three differentcells having the hole transportation layer of the thickness of about300, 500 or 700 Å.

Namely, a 300, 500 or 700 Å-thick α-NPD, a 600 Å-thick layer includingAlq:C545T=100:1 (weight ratio), and a 100 Å-thick mixed layer includingbathocuproine and a metal cesium (Cs) were sequentially deposited oneach substrate to form a light-emissive unit 3-2. Subsequently, V₂O₅(vanadium pentaoxide) was deposited at a deposition rate of about 2Å/sec to form a charge generation layer 4-2 having a thickness of about300 Å.

After formation of the charge generation layer 4-2, the above-describedprocess was again repeated. That is, a 300, 500 or 700 Å-thick α-NPD, a600 Å-thick layer including Alq; C545T=100:1 (weight ratio), and a 100Å-thick mixed layer including bathocuproine and a metal cesium (Cs) weresequentially deposited on the charge generation layer 4-2 to form alight-emissive unit 3-3.

Finally, aluminum (Al) was deposited through a metal mask 41 for cathodelayer formation (see, FIG. 10C) at a deposition rate of about 10 Å/seconto the light-emissive unit 3-3 to form a cathode electrode 5 having athickness of about 1,000 Å. The organic EL device having a squarelight-emissive area of 0.2 cm (length) by 0.2 cm (width) was thusobtained (see, FIG. 10D).

In the resulting organic EL device, a DC voltage was applied to betweenthe anode electrode (ITO) and the cathode electrode (Al) to measure thecharacteristics of the green light emitted from the light-emissive layer(co-deposited layer of Alq and C545T). The results plotted in FIGS. 27,28 and 29 were obtained. In these figures, the symbols ◯, □ and + eachrepresents a graph of the luminance (cd/m²)−voltage (v) characteristiccurve, a graph of the current density (mA/cm²)−voltage (v)characteristic curve, and a graph of the current efficiency(cd/A)−current density (mA/cm²) characteristic curve, respectively, ofeach of the EL devices having the three different thickness describedabove.

As shown in FIG. 29, the EL devices having the three different thicknesseach have a largely varied current efficiency (cd/A). In the deviceshaving the light-emissive units 3-2 and 3-3 having a thickness of about700 Å in the hole transportation layer, a maximum current densityexceeding about 48 cd/A was obtained, while in the devices including thelight-emissive units 3-2 and 3-3 having a thickness of about 300 or 500Å in the hole transportation layer, the current density obtained wasonly about 18 or 28 cd/A.

The EL device including the light-emissive units 3-2 and 3-3 having athickness of about 700 Å in the hole transportation layer show that theyhave a current efficiency of about 16 cd/A (48/3 cd/A) per alight-emissive unit, and thus they represent the optimized examples inwhich in all of the three light-emissive sites, an optical path length(product of a real layer thickness and an index of refraction) from thelight-emissive site to the Al cathode (light reflective cathode) isalways approximately an odd-numbered times a quarter wavelength oflight, i.e., in this example, the layer thickness is ¼ wavelength, ¾wavelength and 5/4 wavelength of the emission wavelength, respectively,from an Al cathode side of the device.

An emission spectrum of each of the three organic EL devices obtained inExample 4 is shown in FIG. 19. Furthermore, the emission spectrum of thedevice showing the maximum current efficiency (48 cd/A), selected fromall the emission spectrums of the devices of Example 4, is also plottedin FIG. 16 for the comparison with the spectrum of the device (onelight-emissive unit) of Reference Example 1 and the spectrum of thedevice (two light-emissive units) of Example 1.

Example 5

(Example for the Production of the Organic EL Device Including TwoLight-Emissive Units Having Different Emission Spectrums; Experimentsfor Optimizing the Optical Path Length, a Distance from EachLight-Emissive Site to a Reflective Electrode.)

Three sheets of the ITO pattern-coated glass substrate were provided,and in accordance with the process which is substantially the name asthat of Example 3, a blue light-emissive unit and a red light-emissiveunit were deposited through a V₂O₅ (vanadium pentaoxide) of thecharge-generation layer 4-1 with the proviso that, in this example, forthe purpose of ascertaining the optimum conditions for an optical pathlength of from a blue light-emissive site of the light-emissive unit 3-1to the light reflective electrode, a layer thickness of the holetransportation layer including α-NPD of the light-emissive unit 3-2 wasvaried with intention to obtain three different cells having the holetransportation layer of the thickness of about 300, 500 or 700 Å. Otherlayer deposition conditions and measurement conditions are with the sameas those of Example 3.

In FIGS. 30, 31 and 32, the symbols □, + and ◯ each represents a graphof the luminance (cd/m²)−voltage (v) characteristic curve, a graph ofthe current density (mA/cm²)−voltage (v) characteristic curve, and agraph of the current efficiency (cd/A)−current density (mA/cm²)characteristic curve, respectively, of each of the EL devices having thethree different thickness obtained in this example.

Furthermore, an emission spectrum of each of the three organic ELdevices obtained in this example (Example 5) is shown in FIG. 20.

As shown in FIG. 32, the EL devices having the three differentthicknesses each had a largely varied current efficiency (cd/A). In thedevices including the light-emissive unit 3-2 having a thickness ofabout 700 Å in the hole transportation layer, a maximum current densityexceeding about 8 cd/A was obtained, while in the devices including thelight-emissive unit 3-2 having a thickness of about 300 or 500 Å in thehole transportation layer, the current density obtained was only about6.5 or 4 cd/A.

The EL device including the light-emissive unit 3-2 having a thicknessof about 700 Å had an optical path length (product of a real layerthickness and an index of refraction) from the light-emissive site ofspiro-DPVBi (blue light-emissive material) to the Al cathode (lightreflective electrode) of about three times a quarter wavelength oflight. Namely, the EL device is an example of an optimized device.

Example 6

(Example for the Production of the Organic EL Device in which a LayerContacting a Charge Generation Layer on an Anode Side is an In-SituReaction Generating Layer, and Having a Charge Generation LayerConsisting of the Mixture of V₂O₅ and Arylamine Compound.)

The organic EL device according to the present invention having alaminate structure shown in FIG. 41 was produced as follows.

In accordance with the manner which is substantially the same as thatdescribed in Reference Example 1, a light-emissive, unit 3-1 wasdeposited through a metal mask 40 for organic layer formation (see, FIG.10B) on an ITO pattern-coated glass substrate 1 shown in FIG. 10A.Namely, a 600 Å-thick α-NPD and a 700 Å-thick layer including Alq:C545Ta 100:1 (weight ratio) were sequentially deposited. Thereafter thein-situ reaction generating layer was formed thereon.

Namely, a metal-organic complex of 8-quinolinolato lithium (hereinafter,briefly referred to as “Liq”) represented by the following formula

was deposited by 10 Å. Thereafter Al was deposited as a thermallyreducible metal at a deposition rate of about 1 Å/sec to form an in-situreaction generating layer having a thickness of 15 Å.

Subsequently, V₂O₅ (vanadium pentaoxide) and α-NPD was co-deposited at amolar ratio (V₂O₅:α-NPD=4:1) on the in-situ reaction generating layer ata deposition rate of 2 Å/sec to form the charge generation layer 4-1having a thickness of 200 Å. The charge generation layer was alsodeposited via the metal mask 40 for organic layer formation (see, FIG.10B).

Thereafter, while the metal mask 40 for organic layer formation (FIG.10B) is still on the glass substrate 1, the above-described step wasagain repeated to form a light-emissive unit 3-2. Namely, a 600 Å-thickα-NPD, a 700 Å-thick layer including Alq:C545T=100:1 (weight ratio), anda 10 Å-thick Liq were sequentially deposited. Finally, aluminum (Al) wasdeposited through a metal mask 41 for cathode layer formation (see, FIG.10C) at a deposition rate of 10 Å/sec to form a cathode electrode 5having a thickness of about 1,000 Å. The organic EL device having asquare light-emissive area of 0.2 cm (length) by 0.2 cm (width) was thusobtained (see, FIG. 10D).

In this organic EL device, a DC voltage was applied between the anodeelectrode (ITO) and the cathode electrode (Al), and the characteristicsof the green light emitted from the light-emissive layer (co-depositedlayer of Alq and C545T) were measured to obtain the results in FIGS. 42,43, 44 and 45. In FIGS. 42, 43, 44 and 45, the circle symbols (●)represent a graph of luminance (cd/m²)−voltage (v) characteristic curve,a graph of current density (mA/cm²)−voltage (v) characteristic curve, agraph of current efficiency (cd/A)−current density (mA/cm²)characteristic curve and a graph of luminous efficiency (1m/W)−luminance(cd/m²) characteristic curve, respectively, of the EL device of Example6.

For comparison, a result of a reference device (ITO/α-NPD, 600Å/Alq:C545T=100:1, 700 Å/Liq, 10 Å/Al) having a conventional structurewas plotted in FIGS. 42, 43, 44 and 45, using the circle symbols (◯).

As shown in the drawings, in the organic EL device in which thelight-emissive unit was partitioned into 2 units, a maximum currentefficiency (and quantum efficiency) is improved to twice as the organicEL device in the above reference device.

In the EL device of Example 6, it is considered, like in Example 1, thata charge transfer complex (V₂O₅ ⁻+α-NPD⁺) was formed between moleculesof the V₂O₅ and the α-NPD, a hole transporting arylamine molecule, by anoxidation reduction reaction. A mixed layer of the V₂O₅ and the α-NPD isfunctioned as the charge generation layer.

Furthermore, in this EL device, a material constituting the in-situreaction generating layer includes only the organic metal complex havingan alkaline metal ion (Lithium ion in the EXAMPLE 6). The material canbe, however, a mixed layer of the electron transporting compound such asbathocuproine and Alq and the organic metal complex (see JapaneseLaid-open Patent Application No. 2000-182774) or a layer including theorganic metal complex containing one of said metal ion.

The in-situ reaction using inorganic compound containing one of saidmetal ion can also be adopted for the layer contacting the chargegeneration layer on an anode side, because such in-situ reaction hasbeen observed conventionally also in using an inorganic alkaline metalcompound as a contacting material to Al cathode and so on (see areference document “J. Endo, T. Matsumoto, and J. Kido, Jpn. 3. Appl.Phys. Vol. 41 (2002) pp. L800-L803”).

Test Example

(Measurement of Resistivity in the Charge Generation Layer and the Like)

In this example, the resistivity (Ω cm) was measured with two differentmethods depending on the range of the resistivity of the test sample.

The first measuring method can be suitably applied to a test sampleshaving a relatively large resistivity. The measurement is carried out bysandwiching a vapor deposition layer of the test sample with electrodes(see FIGS. 33 and 34). The resistivity of the test sample is thencalculated from a ratio of the electric field E(V/cm), obtained from anapplied voltage (V) and a layer thickness (cm) of the deposition layerof the sample, i.e., distance between the electrodes, and a currentdensity (A/cm²) obtained from an observed current value (A) and across-sectional area (cm²) of the current flowing region, i.e.,resistivity (Ω cm)=(V/cm)/(A/cm²).

The resistivity evaluation device for use in this measuring method canbe produced in accordance with the following method. FIG. 33 is a planview of the evaluation device, and FIG. 34 is a cross-sectional viewthereof.

As in the Examples and the Reference Examples described above, a metalmask 40 shown in FIG. 10B is used. A test sample (material, aresistivity of which is intended to be measured) 18 is deposited,through a shadow mask for forming both an organic layer and a chargegeneration layer, at a desired thickness on an ITO electrode 16 having awidth of about 2 mm or, alternatively, an aluminum electrode having awidth of about 2 mm. Finally, an aluminum electrode 17 having a width ofabout 2 mm is deposited in such a manner that it is crossed with the ITOelectrode 16. A desired evaluation device is thus obtained.

The second measuring method can be suitably applied to the test sampleshaving a relatively small resistivity. The measurement is carried out byusing a resistivity evaluation device having a coplanar arrangementstructure. Namely, as shown, in FIGS. 35 and 36, a substrate 19 isprovided, and on the same plane surface of the substrate 19, electrodeswhich are used as an anode 20 and a cathode 21 are previously depositedat a certain distance of L cm. A test material 22 is deposited through ametal mask for defining a deposition area having an opening with thecertain width (W cm) on the electrodes-deposited substrate 19 to obtaina deposited layer having a desired thickness (t cm). In this measuringmethod, an electric field E(V/cm) of the test sample is calculated bydividing an applied voltage (V) with a distance (L cm) between theelectrodes, and a current density (A/cm²) is calculated by dividing anobserved current value (A) by a cross-sectional area of the currentflowing region (in this example, W×t cm²). The resistivity (Ω cm) of thetest sample can be calculated from the equation described above withregard to the first measuring method (sandwich method).

FIG. 37 is a graph showing the measurement results of the resistivity.The test samples used herein are ITO (transparent electrode material),V₂O₅ (a charge generation layer according to the present invention), aco-deposition layer of V₂O₅ and α-NPD (three kinds of molar ratios ofV₂O₅:α-NPD=4:1, 1:1, 1:2) (a charge generation layer according to thepresent invention), a co-deposition layer of V₂O₅ and 2-TNATA[V₂O₅:2-TNATA=4:1 (molar ratio) (a charge generation layer according tothe present invention)], a co-deposition layer of α-NPD, Cs andbathocuproine [Cs:bathocuproine=1:1 (molar ratio)(electron injectinglayer in the light-emissive unit)], and Alq (light emitting material).

For the ITO, the co-deposition layer of V₂O₅ and α-NPD, and theco-deposition layer of V₂O₅ and 2-TNATA, the resistivity was measuredusing a measuring device having a coplanar arrangement structure(coplanar arrangement method), and for the α-NPD, the co-depositionlayer of Cs and bathocuproine, and Alq₃, the resistivity was measuredusing a measuring device having a sandwich structure (sandwich method).Furthermore, the α-NPD having a thickness of 1000 Å was measured by ameasuring device having the sandwich structure wherein the mixed layerof V₂O₅ and α-NPD (the composition of the charge generation layeraccording to the present invention) was formed thinly by 50 Å on aportion contacting both electrodes to make the charge injection from theelectrode ohmic.

Furthermore, with regard to V₂O₅, the resistivity thereof was measuredby using both of the coplanar arrangement method and the sandwich methodwith the result that a substantially same resistivity can be measuredregardless of the difference of the applied methods.

Coplanar Arrangement Method:

◯ ITO 4.6×10⁻⁴ Ωcm

● V₂O₅ 7.2×10^($) Ωcm

▴ co-deposition layer of V₂O₅ and α-NPD (V₂O₅:α-NPD=4:1) 2.0×10³ Ωcm

⋄ co-deposition layer of V₂O_(Z) and α-NPD (V₂O₅:α-NPD=1:1) 3.6×10⁴ Ωcm

+ co-deposition layer of V₂O₅ and α-NPD (V₂O₅:α-NPD=1:2) 2.9×10⁵ Ωcm

□ co-deposition layer of V₂O₅ and 2-TNATA (V₂O₅:2-TNATA=4:1) 5.8×10³ Ωcm

Sandwich Method:

Δ ITO/V₂O₅/Al 2.8×10⁵ Ωcm

▾ ITO/α-NPD/Al 1.5×10¹³ Ωcm

▪ ITO/V₂O₅:α-NPD (50 Å)/α-NPD (1000 Å)/V₂O₅:α-NPD(50 Å)/Al 8.0×10⁸ Ωcm

X Al/Alq₃/Al 6×10¹³ Ωcm

| ITO/Cs:bathocuproine/Al 2×10⁵ Ωcm

FIG. 40 shows a relationship between a mixed ratio (molar fraction) ofthe co-deposition layer of V₂O₅ and α-NPD, and resistivity. As shown inFIG. 40, due to the mixing of both materials, the charge generationlayer according to the present invention indicates a resistivity lowerthan that of each material. This result suggests a presence of oxidationreduction reaction caused by the transfer of electrons, i.e. formationof the charge transfer complex. Accordingly, it was found that theresistivity of the charge generation layer can be varied depending theway of contacting the electron accepting material such as V₂O₅ with thehole transporting material, using an appropriate method such aslaminating or mixing.

As described above, since the EL device of the present invention has astructure wherein two or more light-emissive units were arranged betweenthe electrodes while the light-emissive units are partitioned with anelectrically insulating charge generation layer, an EL device having along operational lifetime and a high luminance region can be achievedwithout increasing a current density so much. Furthermore, it is notnecessary to frequently change and to precisely position shadow masksfor defining a vapor deposition area during production, especiallyduring the formation of two or more light-emissive units and a chargegeneration layer. Furthermore, in the production of simple matrix-typedisplay-devices, it is not required to perform an operation which maycause a risk of disconnection in formation of a cathode line, thusenabling to retain high productivity, and effectively and stably producean organic EL device with a high luminance and long operationallifetime.

Moreover, when the EL device was applied to the production of anillumination apparatus, since a voltage reduction due to the resistanceof the electrode material can be diminished, it becomes possible toachieve an uniform light emission over a large surface area. Similarly,if the EL device was applied to the production of a display devicehaving a simple matrix structure, since a voltage reduction due to thewiring resistance and an increase of the substrate temperature can belargely diminished, it becomes possible to achieve a large surface areasimple matrix display device which could not be obtained using theconventional EL devices.

Obvious changes may be made in the specific embodiments of the presentinvention described herein, such modifications being within the spiritand scope of the invention claimed. It is indicated that all mattercontained herein is illustrative and does not limit the scope of thepresent invention.

What is claimed is:
 1. A display device comprising an organicelectroluminescent element comprising a plurality of light emissiveunits, each of which has at least one light emissive layer, providedbetween a first electrode and a second electrode opposed to said firstelectrode, wherein said light emissive units are partitioned from eachother by at least one charge generation layer, wherein said chargegeneration layer comprises an electron accepting material and anelectron donating material and has a resistivity of not less than1.0×10² Ωcm, wherein the organic electroluminescent element isconfigured to emit light from an area where the first electrode and thesecond electrode are vertically superimposed, and wherein the areacomprises a plurality of areas, each of the plurality of areascorresponding to pixels of the display device.
 2. The display deviceaccording to claim 1, wherein said charge generation layer has aresistivity of not less than 1.0×10⁵ Ωcm.
 3. The display deviceaccording to claim 1, wherein said charge generation layer comprises alaminated layer of said electron accepting material and said electrondonating material.
 4. The display device according to claim 1, whereinsaid charge generation layer comprises a mixed layer of said electronaccepting material and said electron donating material.
 5. The displaydevice according to claim 1, wherein said light emissive layer containsa fluorescent material or a phosphorescent material.
 6. The displaydevice according to claim 1, wherein a plurality of said light-emissiveunits have mutually different emission spectrums.
 7. The display deviceaccording to claim 6, wherein said organic electroluminescent element isconfigured to emit white light due to superimposing of differentemission spectrums of each light-emissive unit.
 8. The display deviceaccording to claim 1, wherein said charge generation layer has a visiblelight transmittance of not less than 50%.
 9. The display deviceaccording to claim 1, wherein said organic electroluminescent element isconfigured to emit light only from where the area where said firstelectrode and said second electrode are vertically superimposed.
 10. Thedisplay device according to claim 1, wherein said electron donatingmaterial comprises an arylamine compound, wherein said arylaminecompound is represented by the following formula (I):

wherein Ar₁, Ar₂ and Ar₃ each independently represents an aromatichydrocarbon group which are allowed to have substituents.
 11. Thedisplay device according to claim 10, wherein said arylamine compoundhas a glass transition temperature of not lower than 90° C.
 12. Thedisplay device according to claim 10, wherein said arylamine compoundhas an ionization potential of less than 5.7 eV.
 13. The display deviceaccording to claim 1, wherein said electron accepting material comprisesan organic material having at least one cyano group or fluorine group.14. The display device according to claim 1, wherein said electronaccepting material comprises an organic material having both at leastone cyano group and at least one fluorine group.
 15. The display deviceaccording to claim 1, wherein said first electrode comprises an anodeselected from ITO (indium tin oxide) or IZO (indium zinc oxide).
 16. Thedisplay device according to claim 15, wherein said second electrodecomprises a cathode selected from ITO (indium tin oxide) or IZO (indiumzinc oxide).
 17. The display device according to claim 15, wherein saidsecond electrode comprises a cathode selected from Al (aluminum) or Ag(silver).
 18. The display device according to claim 1, wherein saidorganic electroluminescent element comprises an electron injection layeras a layer that contacts with said charge generation layer on the sideof said first electrode, and wherein said electron injection layercomprises an organic material and an electron donating dopant.
 19. Thedisplay device according to claim 18, wherein a thickness of saidelectron injection layer is less than 5 nm.
 20. The display deviceaccording to claim 1, wherein said organic electroluminescent elementcomprises a hole injection layer as a layer that contacts with saidcharge generation layer on the side of said second electrode, andwherein said hole injection layer comprises an organic material and anelectron accepting dopant.
 21. The display device according to claim 20,wherein the thickness of said hole injection layer is less than 30 nm.22. The display device according to claim 1, wherein the first electrodeand the second electrode are both in the form of a strip.
 23. Thedisplay device according to claim 22, wherein the first electrode andthe second electrode are crossed, thereby forming the area where thefirst electrode and the second electrode are vertically superimposed.24. A method of emitting light from a device comprising applying avoltage to an organic electroluminescent element within the device,wherein the electroluminescent element comprising a plurality of lightemissive units, each of which has at least one light emissive layer,provided between a first electrode and a second electrode opposed tosaid first electrode, wherein said light emissive units are partitionedfrom each other by at least one charge generation layer, wherein saidcharge generation layer comprises an electron accepting material and anelectron donating material and has a resistivity of not less than1.0×10² Ωcm, wherein the organic electroluminescent element isconfigured to emit light from an area where the first electrode and thesecond electrode are vertically superimposed, and wherein the areacomprises a plurality of areas, each of the plurality of areascorresponding to pixels of the display device.
 25. The method of claim24, wherein the charge generation layer has a resistivity of not lessthan 1.0×10⁵ Ωcm.
 26. The method of claim 24, wherein the chargegeneration layer comprises a laminated layer of said electron acceptingmaterial and said electron donating material.
 27. The method of claim24, wherein the first electrode and the second electrode are both in theform of a strip.
 28. The method of claim 24, wherein the first electrodeand the second electrode are crossed, thereby forming the area where thefirst electrode and the second electrode are vertically superimposed.